How big is it exactly? I'm assuming you mean it's somewhere around 5 inches...? If that's the case then you aren't much under average and it's nothing to worry about. I mean with an average of about 6" you are pretty normal, and remember that you do have some growing to do still as well.
Are you overweight at all? Only men put on a lot of fat around their abdomen and that can have an effect on penis size. For every ten pounds of fat you put on you can lose another 1/2 inch of your penis length due to the fat that builds up around the outside of the base.
But seriously, you shouldn't listen to whoever is laughing at you. Are they seeing it erect anyway? I mean when it's flaccid it will look a different size to other people's and will even vary depending on how warm you are and the mood you're in. Lots of penises even out size wise when you compare them erect.
Read up in the Sex forum here about penis size - it's a common topic and not something to be embarrassed about. But seriously, i think you sound perfectly normal and you should stop worrying. AT the end of the day it's your sexual technique that counts anyway.