Your friend or his parents might be part of the sovereign citizen crowd who have some rather unusual — and incorrect — notions of the law. A contract need not be "under seal" to be valid.
That said, a contract does not allow one to use physical force, like restraining people, to enforce its terms. If a student were to leave in violation of the contract there might be an action available in the courts for breach of contract, though I can't see what damages the school or dodge ball group would have because of it. They could also seek an injunction from the courts for an order to prevent the kid from leaving again. And that assumes that the contract is enforceable. It might not be. While a contract does not have to under seal to be enforceable, in general contracts are not enforceable against someone under the age 18, with some exceptions.
They might be able to drop the kid from the dodge ball group if he leaves early regardless of the contract.
But blocking them from leaving and physically restraining them from going would generally not be proper. Of course, once they leave, if they are violating the curfew law, those cops at the door could cite or arrest them for the curfew violation.