Flirting with Exboyfriend who is talking to this other girl?

Obviously, feelings still exist between both of you, but this is pretty normal after breakups. But the fact remains: the decision has been made. You broke up and it is time to move on. Seeing each other at school everyday only makes things more difficult, thus your ex is using this other girl as a means to get over you. Unless there is real progress on both sides towards getting back together, your only viable option is to cut him from your life as completely as possible and learn to move on. Yes I know this sounds a bit harsh. But you have to look out for yourself now, and preserving your emotional wellbeing is number one on your list.