There needs to be a balance between reprimanding the wrong actions, and compassion to make sure there is no increase in hate.
I don't think I can make any actual suggestions with regards to that, but both are important. They need to know that there are consequences for their actions, but they shouldn't feel looked down on either.
Educating others about things like gender differences, sexual orientations, racial differences, learning difficulties, and differences in interest (jock, geek, designer, etc), and so on.
There are many reasons behind bullying, and one of them is ignorance. It's dire that bullies know their behaviour is wrong, but they should also be educated to know why.
I was picked on a lot in school, and if there was a teacher around who was ready to stand up for me, the bullies often felt marginalised and that teachers favoured me (they kind of did, because I actually did my homework and paid attention in class). As a result, the bullies liked me even less, and the bullying increased.
Fortunately it wasn't violent, but words hurt even more, and leave a lot less evidence.
All I can suggest is to suspend the bullies, and during that time, have them come in for counseling sessions both without (initially) and with the victim (later).