Or get a teacher or administrator to commit ahead of time to go in. Or ask the police (or even, gasp, a lawyer) for advice on handling the situation. Or allow established procedural methods to determine the legitimacy of witnesses' accounts instead of requiring a sting (this is why the legal system specifically allows cross examination of accusers. It's not good at identifying mistaken witnesses, it is surprisingly good at picking out lying witnesses. Unless said witness is a cop. But that's another discussion).
The key here is that a school (one that teaches minors, colleges have different standards), for various reasons, is not required to offer its students due process. Multiple reports of misbehavior, harassment, and sexual assault should be enough for the school to take steps beyond shrugging its shoulders and saying 'I didn't see anything.' Perhaps they shouldn't just expel him out of hand, but a school district has all sorts of steps available.
Frankly, if a teacher, administrator, or district employee has good reason to think crimes are being committed, or will be committed, at the very least a additional supervision should be put into place, even if they don't think it has reached the point where the police should be contacted. And frankly, given what the news is saying the school already knew, they should have called the police.
