The girl you just called fat? 
She's overdosing on diet pills because of an eating disorder. 
The girl you just called Barbie?
She spent hours applying makeup to her skin condition hoping that someone will notice her aside from her blemishes. 
The boy you just tripped in the hall? 
He's abused enough by his own family at home. 
The boy you just called a crybaby in front of everyone? 
His mother is dying and he feels he doesn't have anyone to turn to.
Be the one to tell her she looks perfectly fine the way she is. Correct those who her call her fat because in reality their corsets could be blocking the blood flow to their brains. Show her true beauty isn't weighed on a scale, but in the heart. 
Be the one to show her that she'll truly shine for being her natural self. Everyone has blemishes. That's what sets us apart. Show her we're ALL perfectly imperfect. 
Be the one to guide him to the real family he has. You. Take up the responsibility and be the supporting, loving, and caring brother or sister he needs. Show him he IS loved. 
Be the comforting shoulder he has to cry on. We all have or eventually will loose someone we love, but we don't and shouldn't have to suffer through it alone. Show him he ISN'T alone. 
Everyone in this world has been bullied. Taylor Swift, Justin Beiber, your neighbors. Your mom(s) and or dad(s), the stranger walking beside you. Everyone. So even if you believe you're alone, you never truly are.
Everyone in this world has also been a bully at some point. 
There's reasons why everyone does things, even things they aren't proud of.
To the bullies of the world- 
Don't laugh it off or push someone around if they're not cool enough for you. You never know what's going on in their life that's making them act or appear different. Not only that, but also, take a look at your own life. Find the root that's triggering YOUR actions, and don't be afraid to admit when something is bothering you. People will be there to help you too.