In chapter 21 what do the children do to get revenge?

As vacation approaches, Schoolmaster Dobbins grows more and more strict due to the quick advance of "Examination" day when the entire town gathered to watch performances done by the students of St. Petersburg. His lashings grow more and more severe, and the smaller boys vow to seek revenge upon the fierce schoolmaster. Conspiring together with the sign-painters boy (the master had boarded in the sign-painter's family), the group decides to act on the day of Examination Evening, knowing that before the event Dobbins would celebrate by drinking.

When the time finally comes for the geography class to perform, Dobbins rises to the blackboard and begins to draw a map of the United States. The "tittering" in the classroom increases, but Dobbins remains confused as to what the snickering is about. Above his head is a cat suspended by her haunches with a rag tied around her mouth to prevent her from mewing. Clawing at the string, she is suspended lower and lower until in her clawing and moving about, she grasps the teacher's wig in her claws. The boys have finally achieved retribution, for the sign-painter's son has gilded the schoolmaster's bald head!