I don't know if you're referring to Scout's whole first day of school. If you are, the morning part is found in Chapter Two.

On Scout's first day, Miss Caroline Fisher, her teacher, gets upset that Scout knows how to read and write and makes her feel guilty because of it. Then when Miss Caroline notices that Walter Cunningham hasn't brought a lunch, Miss Caroline offers him a quarter to buy lunch, telling him he can pay her back tomorrow. Scout tries to explain to the teacher that Walter can never pay her back or be able to bring a lunch because his poor, large family pays with goods rather than with money. Miss Caroline again gets upset with Scout and hits her hand with a ruler.

Miss Caroline discovers "cooties", or lice, in Burris Ewell's hair. The teacher sends Burris home to wash his hair with lye soap and kerosene. She does not do so kindly and embarrasses Burris as if he were not human and really beneath her contempt. Burris is teased by the other boys, and he lashes out at both his classmates and teacher.

Burris is one of the boys of the town's poorest families, the Ewell's, whose daughter will later accuse Tom of rape.