I would contact the whatever department of Education that issues your license on the matter and ask about such a thing and bring that contract to an attorney to read to see how it may be interpreted in Court.  It seems unreasonable that the principal can just "take your certificate."  I could see how she could file a complaint if she wished to do so and that you may have been reviewed by the state licensing department but I do not see her as having the sole ability to remove your license from you.  Stand firm.  Don't let her intimidate you.  This will be a lesson for you to learn.  Good luck.