Why did Tom wish he was sick?

Tom wishes he were sick when he wakes up on a Monday morning and contemplates a whole five days of sitting still in school ahead of him:

Monday morning found Tom Sawyer miserable. Monday morning always found him so—because it began another week’s slow suffering in school. He generally began that day with wishing he had had no intervening holiday, it made the going into captivity and fetters again so much more odious.

Tom, of all people, loves being active. Also, the school master he has to face is not the kindest person in the world.
Tom has a loose tooth but he knows that won't work, as Aunt Polly will simply pull it. Therefore, he moans and groans and says he has a sore toe, but that doesn't work either. Aunt Polly sees through his stratagem, and, unfortunately for Tom, he lets slip that he has a loose tooth, so she pulls that as well.
Tom does end up late to school. He is going to lie about it, but sees a beautiful new girl sitting in the schoolroom. He doesn't want to lie in front of her, so he tells the truth. This earns him a whipping from the schoolmaster.

This occurs at the beginning of Chapter 6 in The Adventures of Tom Sawyer by Mark Twain.
On a Monday morning, Tom Sawyer lies awake in bed contemplating the start of a new week at school. Wishing he could escape the “captivity and fetters” of school, Tom wishes that he were sick so that he could remain at home. After considering the possible ailments he could claim to have, Tom settles on his sore toe. First, he begins groaning loudly in an effort to attract the attention of his sleeping brother. When this fails initially, Tom shakes Sid, jumps back into bed, and resumes groaning. This humorous episode is meant to show how Tom has feigned illness before, and he is clever enough to know that in order to fool Aunt Polly, he has to make his suffering appear authentic. Tom over-exaggerates, saying his final goodbyes and listing the heirs of his paltry possessions.
His “mortified” toe, however, doesn’t fool Aunt Polly, who immediately realizes Tom’s scheme. After pulling his loose tooth, Aunt Polly sends Tom to school anyway.