The answer to this question can be found if you look at the paragraph prior to the paragraph when we are told that Mr. DePalma is crying. We are given important background information about Mr. DePalma that present him a very unemotional man who is clearly a strong character and will not take any nonsense from students. Consider what we are told about him:
Mr. DePalma, a short, muscular man with slicked-down black hair, was the science teacher, PE coach, and disciplinarian at PS 13. He was the teacher to whose homeroom you got assigned if you were a troublemaker, and the man called out to break up playground fights and to escort angry teenagers to the office.
Given his role in the school as disciplinarian, it is therefore very incongruous that such a man should be crying openly in front of his students. Of course, the fact that he is only serves to emphasise the emotional impact of the assassination of JFK and its significance to America as a whole.