This paper contributes to the literature on the effectiveness of schools. The analysis is unique in using data on the minutes of instruction per day in each of four subjects taught in the public elementary schools in Illinois. Few education production function papers have information on the amount of instruction students receive in a given subject. The results generally indicate that time spent in mathematics and English instruction pays off in terms of improved mathematics and reading test scores. There is evidence that time spent in social studies instruction raises reading and writing scores. Additionally, the data span 3 years so that individual school-specific fixed effects are estimated and found to be strongly significant. Inclusion of these fixed effects has dramatic consequences for the parameter estimates on several variables, in some cases raising and in others reducing the estimated parameter values. (Contains 12 tables and 16 notes.)