Is 150 pound 15 year old girl who is 5'6 overweight?

No. You're on the higher side of normal. You would have to gain about 7 pounds to be considered over weight. And lose 15 to be exactly in the middle. If you want to be normal but nearly underweight you would want to lose 25 pounds. Please dont do more than that. You're healthy and don't need to have a complex.