Harassment is a lot narrower than people tend to think. It restricts people from makings communications that the subject does not want to be a recipient of. It does not restrict people communicating between each other about the subject. Abby following Jane around calling her fat is harassment if it is severe and pervasive. Abby and Betty texting each other, and talking to each other about how Jane is fat is not.
At least as far as criminal harassment goes. Schools can adopt more restrictive policies but the consequences would only be suspension, expulsion, etc. not criminal charges.
The takeway is that harassment laws restrict what people are saying to you, not what they're saying about you among each other.
If the police actually tried to prosecute them for harassment, the question the courts would be asking is, "could Jane reasonably avoid having to hear it?" Harassment is about being able to not be the recipient of communications you don't want to receive. Calling Jane fat on message boards, twitter, in a bar, etc. would all be fair game.
There's more restrictions if Jane is a captive audience. The courts have ruled that certain settings where people can't easily just leave - namely work and school - have greater restrictions. Again, the question the court would be asking is "can Jane reasonably avoid this speech" and it's a much bigger barrier to leave one's workplace than walk out of the bar.
I thought we have general laws around sexual harassment and revenge porn which criminalize the point of harm.
I bring this up because most of the enforcement difficulty seems to stem from a desire to to control behavior which would otherwise be undetectable.
I believe we already have means of enforcement for students sharing pornography of their classmates.