It's been my experience that people will make fun of other people for almost ANYTHING. They'll mock people for their hair color, their skin color, their body shape or size, the way they walk, and whether their body meets some perceived norm.
There is nothing wrong with the size of your penis. You are 13 years old, and your body will change and develop over the next 5-7 years in ways that you may not even begin to contemplate. This will include changes in height, changes in your voice, changes in your muscle mass, and probably changes to your genitals. Stop worrying about how you look now; it's going to change.
Furthermore, most women don't give a rat's patoot about penis size, so I highly doubt that the teasing is going to affect your ability to be sexual with women in the future. (For one thing, high school only lasts so long, and there's always the chance to meet women who've never heard you be teased.)
How are people finding out about the size of your penis, anyways? Unless you're telling them, or showing it off, most people shouldn't know. You could say something like "Dude, how do you know? You been looking up my gym shorts?" when met by a cruel remark, or you could just give them a Glare of Doom and walk off.