I HATE sixth form Watch
Like someone said above, do productive things during form, like homework, or revision. This is a much better use of your time, rather than talking. Even if you feel like everyone is looking at you because you are doing work, they probably really don't care. Also if you show, and know in your mind that you aren't bothered about what they think, then they won't care and neither will you!
I know you do not want to be friends with them any more, but maybe in future weeks, when the grudge between you both begins to settle down, maybe ask them about exams, and other general questions, like 'do we have assembly today?' etc. This will settle the grudge a little more, and then maybe you won't feel so intimidated by them anymore, and can carry on doing your work without worrying.
Always distract yourself from thinking about them. This can be hard, but keep trying. Like watch a movie, focus on things you want to do with your other friends, or go shopping with your family. This will make you less worried about what they think, and make you more confident in yourself.
Although it's hard, you should try and be confident in what you think is right. Just do your work, and you'll do really well, and it's not long before you all go to university.
Hope this helps and good luck