HELP!!!... I have a major crush...?

there's this guy thatliked me last year but I didn't like him back. then i found out he still liked me and I started to like him back. The he didn't like me anymore so I tried to forget about him and move on but I couldn't. I realised today that he likes me again because he kept staring at me and smiling and when he realised I was looking back he looked away and continued talking to his friends. He's in my lunch, english, and we have geometry las t period together and we have to walk to the same genral area cause he's in band and i'm in orchestra. I alos see him in the hallway and after school alot. The sadie hawkins dance is coming up (and for those of you that don't it's where the girls ask the guys) I want to ask him but I can't go. How do I know for sure he likes me and how do I let him know I like him back with out being too obvious. I alos have to becareful cause I know his rfiends very well and I don't want them making fun of me more than already do by screwing something up.HELP