I'm a teacher, and here's how it would go over at our high school.
First of all, I just had to read a whole web-thing about taking cyber-bullying seriously, as a requirement of the district. So that's realistic.
Our principal, before involving parents, would call in the suspect and talk to them at length. I don't know how he does it, but he usually manages to extract confessions even from the toughest students (it's a continuation high school, so the toughest students tend to be the ones wearing ankle bracelets, to give you a sense). He's not going to strong-arm them, either, because a direct confrontation leading to a power struggle never ends well for the person in charge.
Barring a confession or actual proof, he might call the parents and tell them about his concerns, but it's not likely. Parents, 90% of the time, will side with their kid, and if their kid says s/he didn't do it, they will react with outrage towards the principal for suspecting them. So unless you get proof or a confession, there will be no conference.
If you do, then there would absolutely be a conference, although everyone would not be in the conference together. Chances are, the victim and the victim's family would not be there at all, unless the principal opted to do a mediation, which was a popular thing to do in the 90s. I never see it anymore.
Now, it's more likely that the principal and maybe one or two teachers would meet with the offender and his/her parents. They would show all proof, documentation, etc., of the offender's actions. There would be a contract for the offender to sign agreeing to cease all such activity. Consequences would be laid out clearly.
Now, if after this point the offender reoffends, depending on their situation, those consequences might never be enforced. It depends on a lot of possibilities.
1) the victim could take out a restraining order. This would be a big headache for the school, but we actually currently have that situation at our school and it mainly translates to the two kids involved not having any classes together.
2) if the offender is on probation, any kind of aggressive behavior to the victim, whether the original offense that led to probation had to do with the victim or not, would probably lead to the offender spending some time in juvie.
3) if no legal action had been taken prior, it's possible the principal/teachers might not enforce the contract very effectively. It would require more proof that the reoffense was done by the offender, and so forth.
Let me know if you want more information about how all that would work.
As for the principal making an announcement, he might do that, probably at the next assembly, among other announcements. But in a high school, at least, it's unlikely he'd go out of his way to make an announcement to the classes specifically about it. This sort of thing happens all the time, and while they do deal with it as described above, it's rarely seen as something to make a big deal about to the population at large. As the previous poster indicated, it's unlikely that an announcement would have any positive effect on anyone anyway.