Weight Issues - Underweight?

im 15 years old, 5'4" and i only weigh 107lbs. Im also male and all of my friends weigh so much more than i do and ppl always ask me if im anorexic. Im not, i mean i see food and i eat it, im eating constantly and its mostly junk. You would think i would be gaining weight but i have weighed the same for almost 2 years. Its really starting to agitate me because im always getting called scrawny and i want to gain weight to be a normal size, but i cant no matter what i try. If anyone could give me advice and an explanation to why this is happening that would be great.