Students are not reporting bullying in the school because they believe the school will not doing anything about it or even worse they will end up being the target of bullying.
At your school you have to constantly reinforce why it is important to report bullying behavior.
Every week you must remind them that the only way the school will be bully free is if they get involved.
You need to also remind them the school policy and what will happen to the students the choose to have aggressive behavior toward other students.
At one school district they have this clear policy in place:
The identity of a student who reports bullying, hazing or aggressive behavior, as well as those students who provide information during an investigation will remain confidential to the extent possible and to the extent allowable by law.
Only school personnel directly involved in the investigation of the complaint or responsible for remedying any violation will be provided access to the identity of the complaining student(s) and student witnesses, and then only to the extent necessary to effectively deal with the situation.
Hopefully this school district will remind these student many times why it is important to speak up against bullying.