The Tennessee Value-Added Assessment System (TVAAS) uses statistical mixed-model methods to analyze student achievement data and estimate the effects of schools, class size, and teachers. This study examined the relative impact of teacher effects on student achievement, considering classroom heterogeneity, student achievement levels, and class size. Results show that teacher effects are the dominant factor influencing student academic growth, while classroom context variables have little influence. The study concludes that teachers make a difference in student achievement. Educational researchers have long debated the reliability of teacher-effectiveness studies and how they should inform teacher evaluation systems. However, few studies have directly measured the influence of individual teachers on student progress using standardized testing data. Traditional objections, such as confounding educational effects with external factors and non-random student-teacher assignments, have hindered such research. Teachers often argue that classroom context, particularly student ability levels and heterogeneity, affects their ability to influence student learning. Recent developments, like TVAAS, allow for more accurate estimation of teacher and school effects without socioeconomic confounders. TVAAS uses a large database of longitudinal student, teacher, school, and system data to analyze academic gains. The study analyzed 1994 and 1995 TCAP scores for five subjects and three grades, focusing on academic gain (difference in scale scores between years). Thirty analyses were conducted, considering fifteen subject-grade combinations across two sets of school systems. A mixed-model analysis was used to estimate the effects of various variables on student academic growth. The study highlights the importance of understanding how classroom variability interacts with teacher effects in influencing student achievement.The Tennessee Value-Added Assessment System (TVAAS) uses statistical mixed-model methods to analyze student achievement data and estimate the effects of schools, class size, and teachers. This study examined the relative impact of teacher effects on student achievement, considering classroom heterogeneity, student achievement levels, and class size. Results show that teacher effects are the dominant factor influencing student academic growth, while classroom context variables have little influence. The study concludes that teachers make a difference in student achievement. Educational researchers have long debated the reliability of teacher-effectiveness studies and how they should inform teacher evaluation systems. However, few studies have directly measured the influence of individual teachers on student progress using standardized testing data. Traditional objections, such as confounding educational effects with external factors and non-random student-teacher assignments, have hindered such research. Teachers often argue that classroom context, particularly student ability levels and heterogeneity, affects their ability to influence student learning. Recent developments, like TVAAS, allow for more accurate estimation of teacher and school effects without socioeconomic confounders. TVAAS uses a large database of longitudinal student, teacher, school, and system data to analyze academic gains. The study analyzed 1994 and 1995 TCAP scores for five subjects and three grades, focusing on academic gain (difference in scale scores between years). Thirty analyses were conducted, considering fifteen subject-grade combinations across two sets of school systems. A mixed-model analysis was used to estimate the effects of various variables on student academic growth. The study highlights the importance of understanding how classroom variability interacts with teacher effects in influencing student achievement.