Tom, now happy, walks back to school. On the way, he runs into Becky and apologizes for his behavior to her earlier. But she refuses to speak to him and they part angrily. Tom thinks that if Becky were a boy he would "trounce" her. For her part, Becky looks forward to seeing Tom get punished for the ink in spelling book.
Tom's apology to Becky shows that he's already trying to act more adult after his recent experience with Aunt Polly. Becky's retaliation at Tom is so like a boy's in spirit, though, that he can only imagine treating her like one by fighting her physically.
When Becky enters the schoolroom it is empty, and she notices that Mr. Dobbins has left the key in the desk where he keeps a mysterious book that he reads every day. Unable to resist, she opens the desk to discover an anatomy book inside. Just then, Tom comes into the room. Startled, she slams shut the book and accidentally tears a page as she does so.
Becky continues to misbehave, invading Mr. Dobbins' privacy. Mr. Dobbins, on the other hand, is also acting immaturely, as the book he hides in his desk is either a medical book that allows him to indulge in his fantasy of being a doctor or a pornographic book (as the naive Becky might easily mistake a pornographic book as an anatomy book).
Becky throws a tantrum, and blames Tom for making her read the book. He marvels at how girly she is in fearing punishment for her actions. He doesn't console her, but says he also won't tell on her and predicts that her own guilt will give her away.
Though Becky has been misbehaving as a boy might, she still acts like a girl in fearing punishment rather than appreciating it as a sign of bravery.
The lesson begins. Tom notices how troubled Becky looks. For her part, she is torn about whether to turn Alfred Temple in for spilling the ink on Tom's book. Ultimately she says nothing, and when the mess in Tom's book is discovered she lets him get whipped. Tom takes the punishment without complain, and thinks that he may in fact have been the one who spilled the ink.
Tom so rarely worries about punishment that he doesn't even keep track of his own misdeeds. He is proud to take a punishment, for it gets him attention. This implies that he commits wrongs out of vanity rather than malice, making him essentially a social being, unlike Sid, for example.
As the children study, Mr. Dobbins goes to read his mysterious book. Discovering the tear, he demands to know who the perpetrator is. Wanting to protect Becky both from the pain and the shame of getting punished, Tom takes the blame. He receives a flogging and two hours of detention, but Becky is moved by his bravery.
Tom truly acts as a brave hero who saves his lady. And his action is more than just showing off. It shows true caring and maturity. He shows empathy in understanding that getting punished would deeply shame Becky, and so he takes her punishment as his own.