let her know tha its normal and jus advise her if she wants too do it then to doo it in bed
but shes so worried to go to school everyday
Let her know that there is nothing wrong with masturbation. Some people are against it, and others believe it can be a healthy part of anyone's life. However, I would be a little concerned as to why she started masturbating at a very young age. Did something happen, or did she see something. It's common for kids to re-enact stuff that they see, but you should ask your sister about it as well.
P.s. Teenagers are cruel. Middle school is all about making fun of kids. Tell her to make friends, and stick with them. Friends who give her support. And tell her not to worry so much about kids who make fun of people. Just tell your sister that those kids probably do it too, only thing is, the entire school doesn't know about it. Tell your sister to forget those catty remarks, or weird looks she may get.