Hmmm I was kind of like this during my last year in High School as well.
My word of advice is to study your HSC with a few close friends. Share and help each other if you're doing the same subject, or just study together in the local library. Trials and HSC is mainly just memorising essays, nothing more. Even the 'creative writing' in Adv. English should be prepared and memorised. I remember I would actually skip school on days I didn't have much periods and just meet up with a close friend to study.
As for the competitiveness... well it's not entirely a bad thing. Remember, your UAI is scaled according to the performance of others in your school and your overall school rank. If you can't stand them, just ignore them. Your real friends are made in Uni.
Making friends in uni is very easy. I'm not very sociable and I still have a large group of friends to hang with in most classes. These are the people you will have around in the future. The highschool guys who went off to do a Bachelor of Commerce, not so much (just saying
Also one thing about UAIs. Try aim above your intended score. Uni entry is very flexible, but whether it's a bachelor of science or arts at usyd or unsw, try aim for 85. Your UAI isn't everything, but remember to have a goal.
edit: about mirae's view - I suppose uni is the place where you actually meet up with all your lost elementary school friends as well. In fact, one of my closer friends now is from primary school and I haven't seen her for 6years. Just saying, it's really no problem to find friends/old friends in uni lol