Hey Tess,
One of the most helpful things that you can do is to approach the person who is being bullied to find out the severity of the situation. By doing this you are not only showing that someone cares for them you are also befriending them in the meantime. If your friends pay you out for doing this, then you will soon become aware that they are not your true friends, and possibly, you are better off with the newcomer who is only looking for acceptance. However, if the situation is not able to be controlled or aided by you individually it would benefit the person being bullied more if you told a teacher or parent, as they will have more authority in the sitation. Always try putting yourself in the other person's position and imagine what it is that you would want someone else to do.

'Be who you are, and say what you feel, because those who matter don't mind, and those that mind don't matter' - Doctor Seuss. Good luck!
Ash x