OK, from the start I should point out I know many examples of teachers who are bullies (the profession seems to attract them). However, without knowing the context of this complaint, I think you need to be careful.
If this is you being bullied by a teacher, then go and see your student union. There are formal and informal proceedures you can go through. If this is your child, it's sometimes worth looking further into what is actually happening. When I taught I had several complaints made against me by students and their parents about bullying/racism/unreasonable behaviour. In all instances I wish the parents had come to see me first before going in all guns blazing. Every single one of these complaints turned out to be from a student who was a chronic non-attender or one whose behaviour in class was appaling.
It turns out that what the student percieved as bullying was, actually, the conflict that happens when my expectations of appropriate class behaviour conflicted with theirs. I ran a tight ship, behaviour wise. Some students never seemed to get the concept that if they turned up on time, prepared (where on earth do all the pens go?) didn't talk over the top of me, allowed their classmates to speak when appropriate, did their own work rather than bullying the weaker members of the class into doing it then copying it, didn't swear, throw items have tantrums or demand all the attention, then classes could be quite a lot of fun. In these cases, it was worthwhile going through the formal complaint. It's always worth having a long hard look at how you behave in class, just in case there are improvements that can be made.
Without knowing any more about your situation, I would strongly advise you have a long hard talk with your child (if it's them with the problem), get the facts in order then go and see the school/college/university about a formal complaint. Just be prepared to hear some hard facts from the other side though.