It is not okay to hurt someone and that is exactly what you are doing when you bully a person. Even if you are just teasing - it isn't okay to make fun of another person just because you may not like them or they are different.
If a friend of yours wants you to participate in bullying another school mate, they are not your friends. You need to stand up and say no.
If you are sending messages with harmful content, think about the person on the other end of the message. There is a person that is behind the screen and you need to think about their feelings.
Bullying can have serious consequences. There are a lot of teenagers that have committed suicide because of being bullied. Click on the picture below to take you to a link that talks about teens that have committed suicide.
Take a look - it may stop you from bullying a classmate.