What I used to do once I got home from school each day was make a 'to-do list' with boxes that I couldn't check off until I'd done whatever the study I needed to do. This way I knew what I wanted to accomplish that night, and when they're all ticked off, I knew I'd put enough effort in and learnt what I needed to learn. I try'd to make the activities on the to be specific, so say 'I'm going to do exercises 3D, 3E, 3F'. Most of the time the work got done and I understood what I needed to learn. With that being said, you still need to find a time to take a break somewhere. Something I've been doing lately (didn't do in year 12, I should have though), is going for say 45-60 minute walks/jogs. Its a bit of fresh air, you don't get yourself couped up at home all the time, and can just go and refresh yourself. Then when I get back home I can get back into studying with a fresh mind again (plus my fitness is going up a little bit over time).
On the comparing marks and such, if you want to compare, then do it, but if someone else doesn't want to tell you what they got, then thats their decision, and people should (and will unless they're douches) stop asking. For some subjects it wasn't good for me to compare, for others it was but only with close friends which I knew were at similar levels. Although most of the time for spesh and methods everyone found out what I got since I was getting the highest for most of the time..... teachers... anyway. If it really bugs you comparing, then don't and don't let other people make you feel bad for not comparing. Its your decision and your mark, not theirs.
A few of us helped each other out over the whole year (I swear I spent more time in methods classes helping others than actually doing my own work, which I did mostly at home, but then that was kinda practise for tutoring I guess). It depends on the culture of the school you'er at I guess. If people keep to themselves and don't help others, then its hard to form study groups. BUT it only takes a few people to start off doing that, and when your marks go up from helping the others, everyone else may eventually realise and change. The culture needs to shift, its just a matter of starting that shift. There probably is someone there willing to do the same thing you want to do with helping each other, you just have to find them 
EDIT: Oh and as said previously, keep a good sleeping pattern! Don't sacrifice too much sleep! My friends wondered how I could pick up ideas and such so much more quickly than they could, but it was more due to them having very low hours of sleep, and well they couldn't learn efficiently because of this.
I've tried the checklist method, and it did work in Year 11, until I became lazy and just stopped, thinking I could keep track in my head. BIG Mistake! So, I will be doing that again, hopefully.

My marks for my maths subjects too are publicly announced, though I tell the teacher not to. I don't want my marks known by everyone because it actually puts pressure on me to do well. I mean, people begin to have expectations of you, and it becomes difficult to try and live up to that. That's what happened with Further this year, and I'm just dreading seeing the teacher, because she's going to be SO disappointed. :/
Again, with the study groups, everyone is just competitive and doesn't want their ideas copied. I understand that, seeing as I'm scared of that too, but I guess in year 12, the more help you can get, the better. So will try to get my friends to form a study group, even if others refuse to do it.
& I've been trying to form a sleeping habit over the holidays but have failed MISERABLY. Sleep at 2am, wake at 11-1. Awful, I know. But this will surely change with school starting in less than 2 weeks.

And if things get really stressful and you feel as though the wheels are falling off, REACH OUT. To friends, to family, to close teachers, to a professional.
I really can't reach out to family/friends or teachers because my family aren't very open about feelings. (Don't know if that makes sense) We just don't talk- full stop. I can't see the school counselor either because she's probably going to get the family involved and that'll be awkward again.

So I'll probably need to talk to randoms, if I ever do feel the need to talk, which I probably will.
Rule #1 : Never panic
Rule #2 : Remember Rule # 1
Will do, though it is going to be difficult.