Whose side are you on?

I agree. From a business perspective there is a difference between following the letter of the contract and thumbing your nose at the contractee. It sets the stage for a contentious business relationship and will likely cost you in the future.

the student followed the "contract".

The teacher attempted to "amend" the contract, after initiating the contract.

If the teacher advised "subject to change at any time..." then maybe id be more inclined to say well, caveat emptor.

But that didnt happen here.