Touching your lips and neck is always a good thing. Like maybe you could act as if your neck is a little sore, flick your hair off it and run your fingers up one side, down over your shoulder (flash some skin here and shoulders can be very sexy) to your shoulder rotating your head...he'll be thinking of kissing your neck, pulling your hair and mounting you all at the same time!
Sucking a lollipop (classic)
Playing with a necklace, draws attention to your boobs he'll be imagining seeing them and playing with them and he'll also be imagining you playing with them as your wrists skim across your nipples by accident.
It all depends on exactly how dirty you want to get.? You need to be doing one of these things and know that he is watching and then when you turn and catch him looking, lick your lips and turn away again.
another good way is to text him in class. tell him what you want to do with or to him and see how he reacts to that?
That's probably not the best place to be turning a guy on lol, but if you just dress the way you know he likes best (plus hair style) without looking slutty or having the dress code people down your back that would make him notice you whenever he can see you. Eye contact and giving him your favorite facial expressions help too. If you can get near him then touching works great of course. Text little promises of fun to be had or already had.
I send my boyfriend texts throughout the day telling him snippets of the things I'd like to do to him when we're alone. I put my hand on his thigh and move it as high as he lets me get away with. ;] Whenever I kiss him in the hallways, I add something I know turns him on, like biting his lower lip when I pull away, or tugging on his hair, or grabbing his shirt collar and then running my hand down his chest before I leave. It's all in the smiles and giggles when you leave him hanging too, haha. Every time I act like this during school, he's at my house and all over me as soon as he gets the chance. ;]