If there were no contract, just usual or common hours, your employer could clearly have you stay late, regardless of how inconvenient it was for you, and fire you if you won't. But you write that there us a contract; a contract binds both parties, and defines their obligations. If the contract sets your hours, you can't be made to stay past that, unless the contract has some provision or exception to the hours allowing her to keep you late under these circumstances. If not, you should be able to leave when the contract says you can; and if she takes action against you in violation of the contract's terms, you could sue for breach of vontract.