According to standard height/weight charts you aren't overweight.
Your weight not only depends on your height, but also your frame size. Some with a large frame (large bones) should weigh more than someone who is very small/fine boned. Here's a portion of a chart that applies to you:
Height Small Frame Medium Frame Large Frame
5'5" 117-130 127-141 137-155
At 135, you're really not overweight at all, unless you are very small boned, then just barely.
At 15, your body is not fully developed and you should not try to lose too much weight, or lose it too quickly. Getting down to 120 lbs brings you almost to the under weight category. Be very careful losing that much weight, at this point.
I'm 19 years old, im 5'4 and I weight 151lb and my goal is to lose 30lb... my Dr. said that as long as I was between 110 and 130 I was good. And correct if I'm wrong but weight goes by hight.... So I really think that's a good goal... when are you planing to starting and how do you plan to lose those 15 lbs???