Holling's worst subject in school is math.
Holling does not admit that, but chapter one makes it clear to the readers that Holling struggles in math more than he struggles in other subjects. Mrs. Baker is not happy that she and Holling have to spend Wednesday afternoons with each other, so she looks for a way to get out of it. Mrs. Baker makes a request to the principal that Holling should retake sixth grade math, because his grade was below average. The sixth grade math class conveniently meets on Wednesday afternoons; therefore, Mrs. Baker would not have to supervise Holling. The principal eventually decides that Holling should not go to the math class, because Holling did indeed receive a passing grade.