Generally speaking, young teenaged boys are still just that - boys. Their hormones are all over the place; they cruise venues looking for a girl to chat up, flirt with, etc., just to get a response. When they get a response from one girl, sometimes, it's such a heady feeling, that they want to feel that again, so they find another girl to chat with and be flirty.
Simply speaking, it's like a mating dance. You find someone, chat with them a bit, see what they're like, try out your moves, your "look", your words. You see what works and what doesn't.
You should watch this sometimes in the school cafeteria - watch both boys and girls and their interaction with each other. Read their body language and you can pretty much tell who likes who by how they're acting towards one another.
I don't put a lot of stock in high school flirtations because that's generally what they are. If you like a boy, talk with him. If not, tell him nicely that you're not interested and move on.
