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.