Falsely accused of bullying and how to deal with it.

Hello everybody,

The problem is is that I believe that I have been wrongly accused of bullying. There's this girl in all of my english classes (drama, IT, Geography, English etc.) who is incredibly slow and says really brain-dead things, eg. We've been studying the mogul empire for 6-8 weeks now and she asked "Are we learning about India or Italy?". Naturally, this really anoyed me as it was wasting the learning time of everyone around us. Also, she has a very annoying personality saying things like "you what mate, you calling me fat?" in a really 'chavy' accent despite nobody calling her fat and she being paper thin. I told her (maybe wrongly, in retrospect) that <Insert brain-dead question here> was a stupid question to ask a couple of times. One day, I walked in to geography and sat down. She took one look at me and ran out of the room crying and half an hour later her tutor came back and asked to speak with me outside. She gave me a stern talking to and forced me to admit that I was bullying her despite me not thinking that but did not take any disciplinary action. At the time, I was very afraid of teachers and could not stand up for myself and that is what I regret the most, not standing up for myself. I was literally shaking with fear. Anyway, nothing really happened, for a couple of months I said nothing to her and gritted my teeth when she said anything. But today we had a supply teacher in french and we were on some laptops finishing off work and learning on liguascope. I saw her and her friend playing on random games and discreatly told the teacher. She did nothing, and just said "girls please do your work". This annoyed me because why should they get to play on game whilst every one else does. I told the teacher 3 times and she did nothing. By this time I was really agitated.
So, I walked right up to them and closed the lid of the laptop and said "Why don't you girls just do your work?". This (although I did not know before I did it) shut down the laptop. I went and sat back down to continue with my work. They looked very annoyed and kept on trying to start an argument. I tried to argue my side, but what could I say? They were never going to say "Sorry, you where right" or anything like that. I just said things like "Well, you shouldn't have been playing on games" but they just got more agitated and started saying things like "Why don't you just grow a pair" and started making references to how I 'bullied' her. At this point I decided that the best thing to do would be to just stay quiet, so I did. But they clearly weren't done. They then disrupted the remainder of the lesson for me and my friend by constantly closing the lid of our laptop and making us have to restart our laptop and laughing all the time. Finally, they roped in one of there friends to help them who came up to us a casually started talking to us. Suddenly, he bit me and as I was trying to get him off me he closed the lid of our laptop. By, now it was the end of the lesson so I just left the laptop and put it back in it's trolley and walked out of the classroom trying really hard (and succeeding) to restrain myself from punching all three of them in the face. By the way, I'm a 13 year old boy and in year 8. It's also been really hard for me at the moment as I have fallen out with my main group of friends and I think that it is it for me and them. Thank you very much for reading this, I would be very grateful if you could give me any advice on what to do next. I feel like that neither of us could report the other one because we've both done things which could be used against us by the opposing party but she might do it anyway. Also if it makes a difference our tutors hate each other.