What strikes me if this is the 3rd time he has done such, being a kind of yearly ritual, what makes this one different? Why do something now, when it has happened before? Arresting, and any charges of criminal activity are way beyond the scope of punishment, expelling even might be taking it a bit too far, but for a 3rd time, provided something happened the other time I can see that at least.
And yes, we've all done something like this and told are friends, but rarely did we distribute it at school. I think I ma have saw one written list, and never was it handed out to people, its what someone did in study hall that passed between maybe a dozen people.
It isn't libel/slander in the legal sense because its the equivalent of yearbook comments. It is not going to effect them in any capacity relating to their future. Can they not be homecoming queen because someone wrote something about them? Does it in any way effect their advancement in school? Are they excluded from something because of it?
If those girls who were distraught appeared on news with no ways to disguise their identity, they're just getting some screen time and aren't too hurt by it.
Also, if it was released to cheering from the masses, I don't think it matters much to the majority of the school, which is farther distancing the libel case. And no prospective employer of college is going to look at a prank and exclude the named people. They'll check your disciplinary records, GPA, and extracurricular activities and that's it. Everything else that happened in High School has no bearing on your future from an outside perspective. No one in the professional world cares, unless its local and the person who may be hiring you specifically holds it against you, which that person's problem and they're discriminating.
Also, as far as legal systems, I can bet that one of the girls family is connected to local law enforcement somehow. There is no need to make an example of this as a police officer. At most its the cops coming and saying cut it out to scare you. Corruption exists in the US legal system, or rather people exist, and people are just that, people. No one follows the letter of the law exactly no matter their position. Their personal feelings and loyalties pervade their work to some degree, which means they make decisions which are not 100% in line with the letter of the law.