I've been there, done that, and bought the T-shirt, girl...
Stop being so emo. You are only pissed because you have a low self-esteem and virtually no self-worth. Lighten up. You gotta start communicating with some girls that you know. They don't have to be in your class. Besides, maybe those girls are 'gedik' because that's what you judge them to be but in reality they probably think you are some snooty nosed tomboy. People won't mix with you if they think you don't like them or have the same interest as them in the first place. It's the vibes you are giving.
Still in highschool eh? You don't sound much like the college type somehow... I know that it can be painful from where you are standing now. Don't worry about it. Just hang in there and think of college. There you can make friends that really matter. Same thing happened to me. Peer pressure can be such a pain in the ass.
Don't worry about guys in your class. All boys are schmucks before they hit 21 anyways. Some are still schmucks after that but the lucky few can grow to be decent men. Stop trying to hang round them too much. Don't waste your effort on them because it's not easy being 'one of the guys'. And no matter what you do you can't be one of them because you don't pee standing.
You can try making new friends in tuition class. Or talk more here ^__^ There are lots of people here and over in kopitiam corner who just love to reply to random threads... like me XD
Why are you worried about not being pretty anyways? Maybe you haven't had that growth spurt yet. I'd suggest that you go right now and look at yourself in a decently bit mirror. Find one part of your features that you like the most. Yes... not what other people say. The part YOU like the most that you feel is your most prettiest.
Now go and find things to make it nicer. If it's your hands, start experimenting with hand creams and nice manicure techniques. You might not be able to use nail polish but you sure can buff them good. Your eyes? On weekends or after school, play around with some nice eyeliner or mascara

Hair? Pretty it up with some accessories and nice hair styles. Read magazines to learn how to tie it up differently.
Or maybe it's not about beauty. Maybe you can draw really well! Then by all means buy a sketchbook and start drawing! Move that pencil! Or maybe you are good in studies! Who knows?
In the end, it all boils down to you. You gotta love yourself before someone else can love you, girl o.o; Start small. Find something about yourself that makes you proud and happy. Don't think that there is no one in the world who cares.
I believe in you

Keep us posted on your progress.
P/s: Guys like a girl with confidence in herself ^_~