Why did scout from to kill a mocking bird get in trouble with her teacher

Scout gets in trouble because she knows how to read (the teacher cites she's been taught incorrectly), for explaining why Walter Cunningham won't take a twenty-five cent loan for lunch money, and then finally saying straight out that Walter can't afford to pay her back (Scout is slapped with a ruler). Lastly, the children end up bringing Walter home for lunch where Scout is chastised for being rude to a guest. It was a rough day!

Hope this helps!

Well it depends on where in the book but if you are talking about the beginning of the book it is probably when the kid does not have a lunch and scout says that the kids family is poor.