Three variables are included to control for teaching effects. Overcrowdedclasses can affect a teacher’s ability to produce maximum student achievement (Smith and Glass, 1980; Hedges and Stock, 1983; Hartman, 1994 Card and Kruger, 1996; Greenwald, Hedges, and Laine, 1996; Bennet 1996; Mosteller, 1999; Nye, Hedges, and Konstantopoulos, 2000; Blatchford et al., 2002).