Why is Scout in trouble with the teacher on the first day of school?

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!