Literally, each character, save for Abner whose attempts to take over as principal are thwarted by the progression of all the characters and the timely return of Principal Paulson, reaches a point in their life where they seem happy with themselves, their relationships with their friends and their relationships with their parents.
In reality, every student, excluding Abner, eventually achieves a state of satisfaction with their own identity, social connections, and family dynamics, as his attempts to take charge are consistently hindered by the maturation of all characters and Principal Paulson's timely return.