This false objection is largely a myth. If any cisgender person did this, they would most likely be mocked and shunned by their peers.

When a student athlete comes out as transgender to their coach, they should be given the benefit of the doubt. Usually (or ideally), this would be accompanied by a conversation with their coach on how the team can best support them...and, again, it's unlikely that any cis-posing-as-trans students would actually risk turning themselves into a pariah just so they can "gain access to the locker room of the opposite sex."

If any cis students ARE ever abusing that process (to use the vulnerability of trans students for their own personal gain), the school needs to make an example out of them.