Okay...as a teacher, I'm going to toss out my advice:
1. It completely sucks that this situation has happened to your friend. Let's try and get it sorted so you don't create a bad situation for yourself.
2. In the United States if a school refuses to address issues of bullying they can be in serious legal trouble. The problem (often) is that unfortunately sometimes adults don't listen well to children. Usually a parent has to get involved to "motivate" the reluctant school faculty to respond appropriately.
It's too late (probably) to address the issues surrounding the bullying your friend received; HOWEVER, if you are being bullied by the same people, please TELL YOUR PARENTS and get them to meet with the headmaster to get it sorted out.
3. Willy is right re: physical confrontation. If the bully comes after you, you are justified (theologically and otherwise) to engage in self-defense. Based on what you've said about the situation, Willy is also right to suggest that you film any and every negative interaction with this kid that you can. Which leads me to...
4. If this kid is bothering you in the halls, or saying things under his breath in class, etc., I have a couple of suggestions. A. Get out your phone and start recording. B. Repeat what he said loudly.
It would look something like this:
Bully in the hall: Oi! You wanker. You know I'm coming after you.
You: Hold on! (gets out phone and starts recording) Can you repeat that? I want to post it on my anti-bullying you-tube site.
The bully will either repeat himself (hey...evil makes you stupid) and then you've got evidence (go you), OR more likely, he'll shut up quickly.
Bully during class: You're such a stupid wanker! You're even weaker than that prat who I got to leave.
You: (in a loud, but not angry voice) WHAT DID YOU SAY? ARE YOU BULLYING ME AGAIN BY CALLING ME A WANKER? YOU KNOW THIS SCHOOL DOESN'T SUPPORT BULLYING.
When/if the teacher gets onto you for "being disruptive," smile and say, "I just wanted to make sure we were all following the policies, Mr. So-and-So. I apologize for being too enthusiastic."
It would only take ONCE of a student shouting out in such a manner for me to get down to the bottom of the situation.
LASTLY...
5. Hang in there! I'm praying for you. It totally does get better!