Mexican American educational aspirations and achievements were studied to determine why they tended to be considerably lower than those of Anglos and also to account for the sources of variation within each group, thus attempting to explain the particular contribution of school contexts of varying socioeconomic level and ethnic composition. A survey was taken of 6th, 9th, and 12th grade pupils in the predominantly Mexican American areas of the Los Angeles School District to determine educational patterns and to verify findings. Cumulative school records and questionnaires administered by the staff supplied the data. Academic ability differences between the 2 ethnic groups as measured by achievement tests were found to be the direct result of the teaching provided by the school. Further results indicated the following sources of influence on pupil performance: (1) family educational level was the most important for both groups, with family economic level contributing less; (2) pupil attitudes and values were important for both groups at all grade levels; (3) social context of the school contributed substantially to the performance of Mexican Americans at the elementary and junior high levels and minimally at the senior high level; and (4) English usage made a positive contribution for Mexican American pupils at all grade levels. (CM)