Murphy’s latest entry in his popular MathStart series of easy-to-read math concept stories focuses on the 100th day of school celebration often observed in elementary school classrooms. At the beginning of the school year a group of five children at first think the idea is 100 days of cool (rather than school), so they arrive for the first day in wild costumes and funny glasses. They rather like their misconception and the resulting esprit de corps, so they resolve to continue their pursuit of cool by sporting a different distinguishing trait each day. They try wearing special clothing, dying their hair, walking backward, decorating their bikes, and volunteering as a group, leading up to a celebratory party on day 100. A number line at the top of each spread tracks the progression of days toward 100, with additional conceptual points about fractions inserted in speech-balloon comments. The cool/school word play wears thin quickly, and the efforts of the group aren’t particularly novel or funny. Bendall-Brunello does his best with watercolor-and-pencil illustrations that attempt to make the multi-ethnic group of kids lively and peppy, but this effort remains lukewarm. (author’s note) (Picture book/nonfiction. 6-8)