Surreptitious recording is absolutely legal if either party to a conversation or interaction is aware of the fact.
There’s a cliquishness behind what would be buffoonery if it weren’t for official heavyhandedness going on here, that says the bullies and school administrators involved, as well as the police officer, belong to some group to which the bullied student does not.
In your state, perhaps. Not in all states. Some states require that ALL parties to the conversation give approval to record.