Believe me, I had poor self-esteem before too.

With what you stated, asked yourself these questions first.

*What is your definition of pretty?
*Why are you comparing yourself to your other classmates?
*Does comparing bring you any good?

Having a poor self-esteem came from having a poor self-image of yourself. You need to change on what you think of yourself and replace it with good ones. Do not think about the world view of pretty. I tell you, this world is crooked, so don't believe on what it's definition of pretty. Look into the mirror. Find your good assets physically (I assure you, there is many. But if you can see only one [as you have a difficulty seeing it because of your poor self-image, that's okay]. Do not lie to yourself.

You are still young but as you get mature, you will realize that inner beauty is more important than the physical. I've been there