An endearing third grader, labeled ""obstreperous"" by his teacher, earns a prize for ""best improved conduct"" at the year's end. When lively Lucas Cott bets a classmate that he can seal his lips for a whole school day, he is surprised by the positive results. Through this and a series of other humorous situations, culminating in pinch-hitting as ringmaster instead of literally playing clown for the class circus, Lucas learns to channel his mischievous intelligence. Hurwitz has a sensitive understanding of childlike logic and classroom politics and a gift for the comic (a trip to the barbershop during which Lucas outwits his rambunctious two-year-old brothers is hilarious). As in her previous books, Hurwitz here demonstrates her Cleary-like gift for making the everyday stuff of real life both funny and touching. Hamanaka's ebullient black-and-white illustrations add to the fun.