Miss Caroline's reason for asking Burris Ewell to leave class was his having head lice (kids called them "cooties").
Miss Caroline said desperately, "I was just walking by when it crawled out of his hair...just crawled out of his hair--"
Burris was not upset by his infestation of lice and simply killed the creatures as they revealed themself. However, Miss Caroline was concerned with the other children remaining free of lice.
Despite the fact that Miss Caroline excused Burris for the remainder of the day, she was not his real reason for leaving. He maintained that he "was on the verge of leavin'--I done done my time for this year." According to one of the oldest students, there were many Ewell children attending that school and each of them attended only the first day of school each year. Between his expectation of coming to school only that day and Miss Caroline's instruction to leave, Burris found no obstacle preventing him from abandoning his first grade education.