Bullying is the most common type of relational and interpersonal brutality teenagers in school and school-based or related activities. Most schools or school related activities have shown only a slight or limited success in reducing students or those in a school based setting’s behavior and attitude.
The others don't make any sense and you could do the process of elimination so you would eliminate lesser and few so then your left with the words less and fewer but in this case fewer makes the most sense.