Jem was supposed to take Scout to school, and Atticus paid him to do it.
Scout starts first grade, and Atticus tells her older brother Jem to take her to school. He is in fifth grade.
Jem condescended to take me to school the first day, a job usually done by one's parents, but Atticus had said Jem would be delighted to show me where my room was. I think some money changed hands in this transaction… (ch 2)
Jem also explains that once they are at school Scout should stay away from him, because “school's different.” Scout and Jem live near the school, and Atticus is a hands-off type of parent. He probably considered it a good opportunity for Jem to take responsibility.
This is also part of the process of Jem distancing himself from his little sister. He is a few years older, and considers himself more mature. As he grows to a teenager, he begins to be more of a mystery to his sister. While they still play together during the summer, Jem would prefer not to be seen with his little sister during the school day. Scout realizes that school is not what she thought.