Seek help from trusted adults, school counselors, or family members. Document everything.
StopBullying.gov : Official US government resources on identifying and preventing bullying.
This article dissects the anatomy of this trope, analyzes the character of Yuna (the mother), details the methods of the corrupting bully, and finally, explains what "fixed" truly means in this context.
To a predator like Sera, Yuna was a goldmine.