In January, why did the phrase "So dumb" keep going through Holling's mind? What could he have done to relieve his displeasure with himself?
In the chapter titled "January," Holling kept telling himself how dumb he was because he accidentally said something really insensitive to his teacher, Mrs. Baker, and now he feels guilty about it--and she's obviously mad at him about it.
It was when he was talking with Mrs. Baker about how embarrassed he was--someone had colored the newspaper photos of Holling in his tights and feathers on stage and had plastered the photos all over the school--and Holling had told Mrs. Baker, "It's not like it's your picture in the halls, or that you have all that much to worry about."
It was a dumb thing to say. Mrs. Baker's husband is fighting in Vietnam, risking his life every day, while she tries to go on living and working without him. Her situation is clearly much more dire and stressful than Holling's. So telling her that she had nothing to worry about was not just wrong, but insensitive to her situation.
Instead of silently suffering the shame of what he had said, Holling could have immediately apologized to Mrs. Baker. He could have said, "Mrs. Baker, what I said just now was really stupid. I forgot for the moment about your husband fighting in the war. Please forgive me for being so self-centered and insensitive." This would have eased Mrs. Baker's mind--she understands that Holling is just a teenager, after all--and then Holling wouldn't have to spend the month hiding from Mrs. Baker and feeling guilty about what he had said.