Right now the only guaranteed way to stop the bullying is to fire the clique leader
Or you can try improving relationships of the clique leader and the bullied girl. There's a random check every week to stop the bullying that depends on their relationship - 5% if normal, 20% if they are friends, 50% if they are best friends.
Some of the things that improve relationships and that you can affect directly: mentorship, hosting a show together, being in the same row of a single, doing MC at a concert together
Their graduation date updates once a week too, things that affect it: stamina, relationship with you, dating policy, salary satisfaction, depression/injury, how many shows they host, their position in recent singles, pushes