As a guy, if I knew then what I know now, I would stop myself, chat more, go to lunch with, DEFINITELY be more lighthearted, and try to empathize and think more from girls' point-of-view. I was afraid that if I opened up, it would demean me and knock me off of my hero or cool-guy status. Don't be full of yourself.
Being sociable for a guy at that age takes a little practice; you have to work at it. It is important to be light-hearted and make girls smile and feel good about themselves; otherwise, no matter how 'cool' or good-looking you are, you just won't wear well. Not with the right girls. Learn some really dumb jokes.
Invite yourself over to their house to study. I did this only a few times. One time, I don't know what got in to me, I just wanted to be uncharacteristically forward, and went uninvited to the fridge, moved around some of the dishes, hemmed and hawed, and asked her mother if she could make a sandwich out of that cheese, that ham, and mayonnaise please? I found out later they found it hilarious and loved me for it. I wish I did dumb stuff like that more.
Do stupid stuff like going to play miniature golf and just relax. Put your arms around her from behind and say "Here, swing the club like this." just as a dumb joke like your coming on. Go bowling and order a pizza. I'm not talking about your crush or anything, just classmates that you like, could be interested in, or that show interest in you, are friends, or just at random. I don't care what you look like, if you are nice, and can put on a little charm, you would be REALLY REALLY surprised how far that can carry you. This is the kind of socializing I really regret not doing. And I had ample opportunity.
If you are a girl, just smile and work on a guy's ego a little bit, and invite him over for homework. Or to the school library to study. One girl that liked me a lot, but I never noticed, they used to pull next to me with her whole family in the car - I lived near and walked home from school - and they would all chat with me as they drove along with the windows down next to me, laughing. Her eight or nine your old little sister instigated it all and laughed and chatted and chatted with me and they made me feel really good. We did date after that. Guys at that age need their egos stroked, it is a hormone thing.