The Stanford Achievement Test was administered to 2,186 of the 47,045 Montana high school students to analyze whether or not school size is important in determining student achievement, and to analyze factors that affect student achievement and whether those factors vary by school size. The sample included 1,311 sophomores and 875 seniors. To accomplish the first objective, the mean score of each test was calculated for each school, and the score was standardized so that a composite total score could be calculated. The procedure was the same for all respondents. Regression analysis was used to analyze the data for the second objective. Twelve variables were chosen to represent the school, the student, and the student's socio-economic status. The independent variables were regressed against each of the dependent variables for each of four school size categories and for all students sampled. Results indicated that, at the senior level, no significant differences existed in the mean test scores based on school size. However, sophomores of the larger schools scored better than did those from smaller rural schools. Different variables influenced student achievement in the rural schools than in larger urban schools. Students' socio-economic backgrounds had the most influence on their achievement. (CM)