Whatever they said he signed is null and void because minors can’t enter into any contracts...or so I thought.
I would also argue that the signing of the agreement was without consideration on the part of the teacher. All contracts need consideration to be binding.
If the teacher says that she is teaching the class in return, that does not count, because you cannot use a duty which already exists as consideration for an agreement.
I would also aregue that the teacher’s consideration, the promise to teach, has failed, thus constituting a breach of the contract, thus making any counterveiling obligation by the student void.