When you could withdraw and get a refund (or avoid the obligation to pay, if  you did not yet pay) is governed entirely by the agreement or terms under which you took the class. If under those terms you were allowed to escape paying if you withdrew before a certain date and you did in fact withdraw in time, you should  not pay. The law has no opinion on this matter, other than to enforce whatever the terms of the agreement were.If they insist that you pay, your options are to either pay or refuse; and if you refuse, they may send the bill to collections, sue you, and/or report a default to the credit rating agencies. Depending on how much the bill is for, it may be the best choice to pay it, even if you feel you are in the right.