Dd, 14 has been friends with 3 others since age 11. One of the girls has a bee in her bonnet about dd. Well maybe about the others too but I only hear dds woes. She will often put dd down in front of everyone: why would you Wear that? Type of comment re dds outfit. She blanked dd for weeks because dd went to a sleepover this girl wasn't invited to (only dd was). They are in many of the same classes and sit together (not free choice of seating so coincidence) but she will talk over dd to others and act like dd is invisible, if dd does well in class this girl belittles it like oh the teacher made a mistake. Dd says when this girl is happy with her it is like the sun is shining on her, but it is baltic when she is displeased with dd. It is a small school and difficult to avoid someone plus dds other friends are still friendly with this girl. Dd has other friends but at different schools but has not had to deal with this behaviour before. I am at a loss for what to say to dd. I do not understand why this girl is so mean and I am trying to see how dd could be at fault but she is so distressed for such a long time that I cannot see how she could be.