There is also the problem of food. A little boy may carry good food to class which the 'big bully' can't afford and so treat the small boy badly when the boy refuses to share. This is also a practice common amongst girls towards their fellow playmates. In co-educational schools, bullying can happen when a big boy suspects that the small boy has a girlfriend that he has wooed unsuccessfully. The big boy may beat the small boy for him to discourage the little boy.
Teachers can most times intervene and punish the big bully.
Indeed, bullying is such a bad habit and the big boys are sometimes just being very cruel and wicked. It has even made the victims stop going to school, when the teachers can't do anything towards the threat. Sometimes the students or pupils are afraid to report the big bullies and after much interviews by the victim's parents will it be known, and investigations undertaken. This also causes the poor output of the pupil or student. I suggest that bullying must stop in all schools and institutions and this must be taken seriously in all countries.