Get with the school today and let them know if it is not taken care of and stopped you will consider contacting the local media and also the police. Look up in the student handbook to see what the policy is on bullying, at most schools there is a zero-tolerance policy. It is not your problem that one of the other girls may be having family problems. I would also consider enrolling your daughter in a youth self defense class or taekwondo or karate for kids. Our daughter expressed interest in in at age 5 and we enrolled her in a class, she was never bullied but just wanted to do it for fun and exercise. She became a black belt at age 11. She had a very slight build at that age and it did wonders for her self confidence, coordination and taught her lots in the way of personal responsibility, perseverance, and many other things. Most of her friends did not even know then , nor do they know now that she is a black belt. I'm sure doing this would prevent your daughter from being picked on further since she could defend herself. If you do look into this be sure you tell the Master (head teacher) at the martial arts school why you are enrolling her but tell him/her confidentially. THe imartial arts nstructors will often work with children on issues they are having problems with to help overcome them. Good luck but hang tough with school for your daughter. It is your job to proctect her.