News of a treasure hidden in her soon-to-be-demolished school prompts a third grade sleuth to search for clues that were laid down decades ago.Cued by an elderly neighbor's memories of a never completed treasure hunt set up when she was a third grader in 1947, Leila and canine sidekick Nugget rush to investigate. Though it seems impossible that clues so old could have survived, the authors stretch things surprisingly little to accommodate the unlikely premise as they plant hints for Leila to uncover, ranging from an altered old class photo to origami animals slipped into the spines of certain books. Efforts to keep her hunt secret not only leave her (temporarily) on the outs with her best friend, Kait, but quickly prove futile, and soon several grown-ups, many of her schoolmates, and even a local TV news crew are following behind or haring off on searches of their own. As it turns out, there's no gold (as Kait excitedly supposes), but the "treasure" that Leila discovers with significant help from Nugget and others in a nicely timed denouement does have enough dollar as well as sentimental value to satisfy young chapter-book readers. Race and ethnicity are left ambiguous in the occasional monochrome illustrations. Nugget, a "cavapoo," is as cute as he is clever.A well-constructed mix of brains and footwork. (Mystery. 8-10)