Start like you do when you want to date someone just make it clear you don't anything serious that being said when you say school and the way you state your question your probably in high school or god forbid middle school put a cap on it or use birth control and if you cant get either shut those legs and lock it up. You don't want to be that girl pregnant in school wobbling to your locker.
But be flirtatious in a sly way no one wants someone who comes on too strong. And you don't want people to know its unwanted attention and pressure. Make sure he's someone you can trust you don't want to have "fun" with a big mouth.