Um...when the janitor goes into clean, he knocks and asks for permission to enter. That he does not barge in actually establishes that the expectation of privacy exists until it is specifically waived by anyone in there, or until it is established that there is no one in there with that reasonable expectation of privacy! In a locker room, everyone has a reasonable expectation of privacy until they specifically waive it.
The reporters will not enter a locker room until, again, invited in. Once the reporters are invited in, the expectation of privacy has been explicitly waived.
The teacher had an expectation of privacy. He or she criminally used that expectation, but again, the right to privacy does not evaporate merely because one is committing a crime.
The girl in the locker committed a crime and should be held to account.
But, again, she did not make the recording at the behest of law enforcement. I don't see why it would not be admitted at the trial of the teacher who also committed a crime and should be held to account.