<PAGE>
                                                                   EXHIBIT 10.55

 
                      SIXTH AMENDMENT TO CREDIT AGREEMENT
                      -----------------------------------

     THIS DOCUMENT is entered into as of January 29, 1999, between MATRIX
CAPITAL CORPORATION, a Colorado corporation ("Borrower"), the Lenders described
below, and BANK ONE, TEXAS, N.A., as Agent for Lenders.

     Borrower, Lenders, and Agent are party to the Credit Agreement (as renewed,
extended, and amended, the "Credit Agreement") dated as of March 12, 1997,
providing for a $8,500,000 Term Loan and a Revolving Facility of up to
$11,500,000.  Borrower, Lenders, and Agent have agreed, upon the following terms
and conditions, to extend the Stated-Termination Date for the Commitments and
modify the amount of Charter-School Debt allowable as Permitted Debt under the
Credit Agreement.

1.  TERMS AND REFERENCES.  Unless otherwise stated in this document (A) terms
    --------------------                                                     
defined in the Credit Agreement have the same meanings when used in this
document and (B) references to "Sections," "Schedules," and "Exhibits" are to
the Credit Agreement's sections, schedules, and exhibits.

2.  AMENDMENT.  Section 8.1(g) of the Credit Agreement is entirely amended as
    ---------                                                                
follows:

               (g) Charter-School Debt.  Debt of Equi-Mor owed to U.S. Bancorp
                   -------------------                                        
          Investments, Inc. (or its successor or assignee), on a short-term
          basis for financing charter-school leases or loans until they can be
          sold in the secondary market, for which Borrower has no direct or
          contingent liability; provided, however, that if Borrower is either
          directly or contingently liable, the amount of such Debt may not
          exceed the lesser of either (i) 10% of Borrower's Net Worth or (ii)
          $5,000,000.

3.  CONDITIONS PRECEDENT.  Notwithstanding any contrary provision, the foregoing
    --------------------                                                        
paragraphs in this document are not effective unless and until (A) the
representations and warranties in this document are true and correct, and (B)
Agent receives counterparts of this document executed by Agent, Lenders,
Borrower, and each other Company named on the signature pages of this document.

4.  RATIFICATIONS.  To induce Agent and Lenders to enter into this document,
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Borrower (A) ratifies and confirms all provisions of the Loan Documents as
amended by this document, (B) ratifies and confirms that all guaranties,
assurances, and Liens granted, conveyed, or assigned to Agent and Lenders under
the Loan Documents (as they may have been renewed, extended, and amended) are
not released, reduced, or otherwise adversely affected by this document and
continue to guarantee, assure, and secure full payment and performance of the
present and future Obligation, and (C) agrees to perform those acts and duly
authorize, execute, acknowledge, deliver, file, and record those additional
agreements, and certificates as Agent or any Lender may request in order to
create, perfect, preserve, and protect those guaranties, assurances, and Liens.

5.  REPRESENTATIONS.  To induce Agent and Lenders to enter into this document,
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Borrower represents and warrants to Agent and Lenders that as of the date of
this document (A) each Company has all requisite authority and power to execute,
deliver, and perform its obligations under this document, which execution,
delivery, and performance have been duly authorized by all necessary corporate
action, require no action by or filing with any Tribunal, do not violate
corporate charter or bylaws or (except where not a Material-Adverse Event)
violate any Law applicable to it or any material agreement to which it or its
assets are bound, (B) upon execution and delivery by all parties to it, this
document will constitute each Company's legal and binding obligation,
enforceable against it in accordance with this document's terms except as that
enforceability may be limited by Debtor Laws and general principles of equity,
(C) all other representations

                                         Sixth Amendment to Credit Agreement
                                         -----------------------------------
<PAGE>
 
and warranties in the Loan Documents are true and correct in all material
respects except to the extent that (1) any of them speak to a different specific
date or (2) the facts on which any of them were based have been changed by
transactions contemplated or permitted by the Credit Agreement, and (D) no
Material-Adverse Event, Default, or Potential Default exists.

6.  EXPENSES.  Borrower shall, subject to a contrary written agreement between
    --------                                                                  
Agent and Borrower, pay all costs, fees, and expenses paid or incurred by Agent
incident to this document, including, without limitation, the reasonable fees
and expenses of Agent's counsel in connection with the negotiation, preparation,
delivery, and execution of this document and any related agreements.

7.  MISCELLANEOUS.  All references in the Loan Documents to the "Credit
    -------------                                                      
Agreement" refer to the Credit Agreement as amended by this document.  This
document is a "Loan Document" referred to in the Credit Agreement; therefore,
the provisions relating to Loan Documents in Sections 1 and 12 are incorporated
in this document by reference.  Except as specifically amended and modified in
this document, the Credit Agreement is unchanged and continues in full force and
effect.  This document may be executed in any number of counterparts with the
same effect as if all signatories had signed the same document.  All
counterparts must be construed together to constitute one and the same
instrument.  This document binds and inures to each of the undersigned and their
respective successors and permitted assigns, subject to Section 12.12.  THIS
DOCUMENT AND THE OTHER LOAN DOCUMENTS REPRESENT THE FINAL AGREEMENT BETWEEN THE
PARTIES IN RESPECT OF THE MATTERS COVERED BY THE LOAN DOCUMENTS AND MAY NOT BE
CONTRADICTED BY EVIDENCE OF PRIOR, CONTEMPORANEOUS, OR SUBSEQUENT ORAL
AGREEMENTS BY THE PARTIES.  THERE ARE NO UNWRITTEN ORAL AGREEMENTS BETWEEN THE
PARTIES.

                     REMAINDER OF PAGE INTENTIONALLY BLANK.
                            SIGNATURE PAGES FOLLOW.


                                          Sixth Amendment to Credit Agreement
                                          -----------------------------------
<PAGE>
 
     EXECUTED as of the date first stated in this Sixth Amendment to Credit
Agreement.

<TABLE>
<CAPTION>
<S>                                      <C> 
MATRIX CAPITAL CORPORATION, as            ONE, TEXAS, N.A., as Agent and a 
Borrower                                  Lender
 

By               /s/                      By              /s/
   -------------------------------           --------------------------------
   David W. Kloos, Chief Financial Officer   Mark L. Freeman, Vice President
</TABLE>


                    U.S. BANK NATIONAL ASSOCIATION,
                    formerly Colorado National Bank,
                    as a Lender


                    By            /s/
                        ---------------------------
                         Andrea C. Koeneke,
                         Vice President


                          1 of 2 Signature Pages to 
                      Sixth Amendment to Credit Agreement
<PAGE>
 
                             CONSENT AND AGREEMENT
                             ---------------------

     To induce Agent and Lenders to enter into this document, the undersigned
jointly and severally (a) consent and agree to this document's execution and
delivery, (b) ratify and confirm that all guaranties, assurances, Liens, and
subordinations granted, conveyed, or assigned to Agent or any Lender under the
Loan Documents (as they may have been renewed, extended, and amended) are not
released, diminished, impaired, reduced, or otherwise adversely affected by this
document and continue to guarantee, assure, secure, and subordinate other debt
to the full payment and performance of all present and future Obligation, (c)
agree to perform those acts and duly authorize, execute, acknowledge, deliver,
file, and record those additional guaranties, assignments, security agreements,
deeds of trust, mortgages, and other agreements, agreements, instruments, and
certificates as Agent or any Lender may reasonably deem necessary or appropriate
in order to create, perfect, preserve, and protect those guaranties, assurances,
Liens, and subordinations, (d) represent and warrant to Agent and Lenders that
(i) the value of the consideration received and to be received by the
undersigned in respect of those guaranties, assurances, Liens, and
subordinations are reasonably worth at least as much as the related liability
and obligation, (ii) that liability and obligation may reasonably be expected to
directly or indirectly benefit the undersigned, and (iii) each undersigned is --
and after giving effect to those guaranties, assurances, Liens, subordinations,
and the Loan Documents, in light of all existing facts and circumstances
(including, without limitation, collateral for and other obligors in respect of
the Obligation and various components of it and various rights of subrogation
and contribution), each undersigned will be -- Solvent, and (e) waive notice of
acceptance of this consent and agreement, which consent and agreement binds the
undersigned and their successors and permitted assigns and inures to Agent and
Lenders and their respective successors and permitted assigns.

<TABLE>
<CAPTION>
<S>                                <C>       
MATRIX FINANCIAL SERVICES          UNITED CAPITAL MARKETS, INC.
CORPORATION and MATRIX FUNDING     
CORPORATION
 
 
By            /s/                  By            /s/
   ------------------------------     ---------------------------
   George R. Bender, President of     Austin Tilghman, President
   each above Company                  
 
 
UNITED FINANCIAL, INC.             UNITED SPECIAL SERVICES, INC.
 
 
By            /s/                  By            /s/
   ------------------------------     --------------------------- 
   Richard Schmitz, Chairman          Linda Preston, President
 
 
VINTAGE DELAWARE HOLDINGS, INC.    FIRST MATRIX INVESTMENT SERVICES 
                                   CORP. and THE VINTAGE GROUP, INC.
 

By            /s/                  By            /s/
   ------------------------------     ---------------------------  
   David W. Kloos, Chairman           Paul E. Skretny, Chairman 
                                      of the Board and Chief Executive 
                                      Officer of each above Company
 
</TABLE>

                            2 of 2 Signature Pages
                      Sixth Amendment to Credit Agreement