What's something flirty to say without being too obvious.. ?

First of all, Mademoiselle, catch this young man off-guard. Remember that (at your age), you have the advantage.

Tell him that you like the way he dresses (if he does dress well). Otherwise, you can tell him he is handsome. If you truly wish to be coy (and a daring flirt), give him a big hug, and then say something like "Mmmm, you feel strong enough to scoop me right up".

Few boys your age would be the wiser. Again, you're still being a lady. Keep this in mind, this may help: A good man always seeks a Lady; so be one. We men are so very predictable...

Play to a man's strengths, and you will always be his weakness.

Give genuine compliments, people can usually tell the difference between 'You look good' and a specific compliment about their personality, etc. So if you say 'your really funny' or something like that, that's a subtle way to show that you enjoy his company. You can compliment his clothing, or if he plays sports you can talk about different plays. Give small boosts to his ego that will also make him want to be around you because your an encouraging force ;)