what do the boys do on Examination evening?

They take off the schoolmastesr's wig with fishing line.

"The group decides to act on the day of Examination Evening, knowing that before the event Dobbins would celebrate by drinking.

On the big evening, the town gathers in their best clothes and the exercises begin. Little boys and little girls recite poems and speeches; Tom Sawyer begins with "Give me liberty or give me death," but stops halfway through, unable to remember his lines. There are reading and spelling exercises, a Latin recitation, and then the young ladies present original compositions. Twain describes the compositions as "nursed" and like "petty melancholy." Each piece was a "wasteful and opulent gush" of moralistic and religious teaching. Twain writes: "There is no school in all our land where the young ladies do not feel obliged to close their compositions with a sermonŠ Homely truth is unpalatable."

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!"

"Now the master, mellow almost to the verge of geniality, put his chair aside, turned his back to the audience, and began to draw a map of America on the blackboard, to exercise the geography class upon. But he made a sad business of it with his unsteady hand, and a smothered titter rippled over the house. He knew what the matter was, and set himself to right it. He sponged out lines and remade them; but he only distorted them more than ever, and the tittering was more pronounced. He threw his entire attention upon his work, now, as if determined not to be put down by the mirth. He felt that all eyes were fastened upon him; he imagined he was succeeding, and yet the tittering continued; it even manifestly increased. And well it might. There was a garret above, pierced with a scuttle over his head; and down through this scuttle came a cat, suspended around the haunches by a string; she had a rag tied about her head and jaws to keep her from mewing; as she slowly descended she curved upward and clawed at the string, she swung downward and clawed at the intangible air. The tittering rose higher and higher -- the cat was within six inches of the absorbed teacher's head -- down, down, a little lower, and she grabbed his wig with her desperate claws, clung to it, and was snatched up into the garret in an instant with her trophy still in her possession! And how the light did blaze abroad from the master's bald pate -- for the sign-painter's boy had gilded it!"