A few boys at the back of the classroom were snickering while the teacher wrote on the board.

snickering 是动词 snicker 的现在分词形式。snicker 的意思是"窃笑、暗笑"，指那种压低声音、偷偷摸摸的笑，通常带有嘲笑或恶作剧的意味。