It was 9:45 a.m. on a Wednesday morning. Jane Reiser's mathematics class in Room 18 was stuffed with sixth- and seventh-graders. There were 32 of them, way above the national class size average of 25. Every seat was filled -- 17 girls, 15 boys, all races, all learning styles. A teacher's nightmare. And yet, despite having so many students, Reiser's class was humming, with everybody paying attention. She held up a few stray socks to introduce a lesson on probabilities with one of those weird questions that interest 11- and 12-year-olds: If you reach into your sock drawer in the...