What should my size be at 15.?

The last time I measured my it was 3 1/2 when erect. When I go to school I am subconscious when we go to P.E. Alot of people I see in gym fill out their boxers and it is completely noticeable and you can tell it is so much bigger than mine. Should I be worried or am I just a late puberty person. By the way im 15.