University of Michigan education researcher Valerie Lee set out to find the answer. She divided a national sample of about 800 public and private high schools into categories by size, and measured the learning of close to 10,000 students during four years of high school. Lee reported in the journal Educational Evaluation and Policy Analysis that regardless of the schools' resources or students' backgrounds, they learned the most at schools of 600 to 900 students. "The category '600 to 900' was where students learn the most," says Lee. "When schools got smaller than that, learning was less, and when schools got larger than that, learning was less and continued to be less as they got larger and larger."