So this is going to give you a response to the boy saying (or sometimes a girl saying, depending on the level of cruelty) "You're ugly." In fandom form. Have fun!

Harry Potter
I'm ugly? You look like Voldemort tried to bless you with beauty! Not even a dementor would kiss you!

Percy Jackson
You're ugly enough to be a mix of a Minotaur and Arachne!

Teen Wolf
OH NO! IT'S A KANIMA!
Wait
That's your face.

The Maze Runner
Really? Because you're so ugly not even a Crank could eat you.

Gravity Falls
Whoa! That mask is almost as scary as Toby Determined's face.
Wait.
That's actually your face?
AHHHHHHHHHHHHHHHHHHHHHHH!

Divergent
You're so ugly people in Amity would run away from you.