It sounds like you are stuck between a hard place and a rock.
What you do next really depends on his girl and how she feels about you. If she genuinely likes you back and you think there may be a chance for you two to get back together, or some remaining sparks from your past romance, then it may be worth pursuing further. However, if she is just yanking your chain, starting drama, and getting you in trouble with her boyfriend (not to mention the school, for kicking his ass), then its probably best that you cut all ties with this girl and move on with your life.
Sometimes girls can be totally evil, and they will lead on an ex-boyfriend simply because they do not want to see that person move on from them. They require the attention and the satisfaction of knowing that person still pines for them. Its horrible, but often true.