Unfortunately, filtering websites does not stop cyberbullying. Additionally, it is almost impossible to stop a teenager from having unsupervised access to the internet. Even if parents don't allow unsupervised access in their own home, most teenagers have smartphones/ipads and it only takes five minutes on the way home from school or in a friend's house for the damage to be done.
A typical scenario might be for somebody to take an embarrassing photo of the victim or start spreading a malicious rumour. Such pictures and messages can be round the whole school (even the whole world) in seconds. It doesn't help if the victim has no access to the internet or can't access the apps being used. What often happens next is that gangs of sometimes unknown people start making comments and/or push and shove or blacklist the victim. There's nothing new about all this, but it can spread a great deal faster than in the past. Sometimes the victim doesn't even see the original photo or picture.
My personal view is that filters and blocking sites give parents a false sense of security. It's far better to encourage a teenager to talk, which is easier said than done if you have a full-blown, moody, monosyllabic teenager on your hands, but I think it's far better to try and build up resilience to bullying. If possible, parents can face the perpetrator(s).
@obieone
Schools have to have a bullying policy by law and, in my experience, some schools are better at dealing with it than others. It really isn't true that teachers don't want to know, but sometimes there is a limit to what they can do, if the bullying is occurring on the way to/from school or at home, which is more than often the case.