ETHETE, Wyo. — On one of the last days of school at Wyoming Indian Elementary School in the spring, Cheryl Williams is playing a game of “8-plus” with Atsa, a first-grader with a round, reflective face under a fuzzy thatch of black hair. These sessions are often playtime for Atsa, but they are always serious for Williams, a teacher who works one-on-one with young children to jump-start their stalled mathematical thinking.