Why is my penis small?

I'm 14 and 7 inches soft, and 8 inches hard. And in circumference it's 4 inches. And I go into my locker room after my gym session is over(we have classes to take altogether with people in the same age group.) and everyone says my penis is small. And I was wondering how I can make my penis longer. And please give serious answers because I just can't stand the humiliation at my gym. So please just give me serious answers..