Just pay it no attention. I was never ashamed of my woody when I was younger. It just happens, especially around that age.
If you're really worried, practice this at home. Look into a mirror, and without breaking a smile of any kind, look directly into your eyes and say "Yep, pretty big, isn't it. Must be to be so noticeable"
Break that out next time anyone points it out to you in any way.
Or, if you're feeling puckish, go with, 'Don't worry, it's not for you." Then grin a really big shit eating grin.
