Group-serving bias means that people tend to give a good understanding of the behavior of their group members, and it is easier to imagine from a bad point of view when explaining the similar behavior of other group members.
According to the above definition, when a head teacher sees the slapstick behavior of students in his class, he will think it is just joke and mischief, and when involved in similar behavior of students in other classes, he will think it is bad quality has group-serving bias.