I had a thread just like this not long ago--I also have a child who is an angel at school and very difficult at home. In fact, she consistently gets recognized for role model behavior at school (she's also a straight A student). Your DD sounds a lot like mine and btw, you sound a lot like me in how you have handled it ("I always tell her that this behavior is not okay, etc, etc, etc, and that she needs to say sorry, and go to her room to cool off. But apparently my technique is not working").
Things have gone from bad to worse recently and I've been changing my approach. Rather than sending her to her room or lecturing her, I've been trying to affirm her and reflect her feelings back without judgement or redirection. This is REALLY HARD for me and I am NOT GOOD at it. However, I'm seeing some positive results. This is the website I've been looking at a lot:
http://www.ahaparenting.com/(Now I sounds like a spammer, but I swear I've been on this site a long time!)
