There is nothing more attractive than a girl with confidence. One that can slip over and be the first to laugh kinda thing lol.
Also, boys need their ego stroked but not in a big obvious worshiping way. Maybe ask him for help with the theory part of your gym class (if you have theory) and he will be pleased that you asked, even if he doesn't know the answer. But don't seem like a total nitwit by not knowing any thing, trust me lol.
Get a group of mixed friends (guys and girls) together and go to the movies or beach or something else and invite him for example "Hi, a big group of my friends and I are going to the beach/movies/park to play football etc and I was wondering if you want to come along." kinda thing. Slowly, as you get to know him, let the size of the group shrink so that it is just you too having fun. Plus, it is a great way to find out what kinds of things he likes to do out of school.
If he says no, then say "that's ok. It won't be as much fun but, you know, we will try." and you must say it with a cheeky roll of the eyes and a big smile and maybe a hand over the heart. This slight touch of self draws his eyes to where he could put his hand but you must do it in a joking yet teasing way. Then you MUST walk away, maybe look back with a grin and keep that confident walk till he is out of sight. Then you can cry with a box of chocolates and a chick flick.
Let him believe that he is chasing you, that he wants you. If he gets the vibe that your crushing on him, it can make it awkward and he will run for the hills or he will think that it is sweet cause he is also crushing on you. That is the tricky part. Let him come to you, it may take a bit but it can happen. Keep inviting him to get togethers every now and then but if he turns you down, don't appear crushed.
Also, try not to ask in front of his mates either cause they will jeer and tease you. Also, their presence could make him say no even if he really does want to go. Please, no love spells on him or it could be really bad and make him turn out to be a stalker. If you do want to do spells, try a confidence one or a love spell on you to get that extra vibe that you are lovable and that you can have any one but you chose him.
that is the best I have got, good luck hun.