I cannot stand so-called 'friends' who are more 'users' than people who care for you. However, there are an array of things to be considered:
1. Moving to another college - is there one? How can you get there? Will it offer the same subjects with the same exam boards that you can carry forward?
2. It's only one more year of your life. I have shut myself off a little, during this last year especially, really concentrating on getting some good grades. Work, as daft as it may seem, will really take your mind off things.
3. To increaase your confidence, throw yourself in the deep end, i.e. join a club or society or something; or why not even get a part-time Saturday job? I lost some confidence once; I like to think that employment has given me some back.
4. People should be supporting you - you did the right thing getting the police involved; anyone calling you a 'grass' ought to be shot!
Remember, you have no reason to have low confidence. We are all (within reason, at least) dignified people. If you keep telling yourself this, it can really make life a more attractive prospect... Really!