The impact of language of instruction (Spanish or English) during elementary school on the subsequent educational achievement of Hispanic students in high school was examined. Data from the "High School and Beyond" study, a longitudinal study of 58,000 high school sophmores and seniors, were analyzed. Three types of elementary school situations were identified: the language of instruction was either predominantly English, mixed English and Spanish, or predominantly Spanish. Students classified as limited or non-English speaking in mixed language classrooms performed better in reading and math than their peers with similar backgrounds in both predominantly English and predominantly Spanish classrooms. Students in predominantly English classrooms had reading achievement scores in the middle range and the lowest math scores. Students in predominantly Spanish classrooms had the lowest reading achievement scores. It is concluded that different languages of instruction have varying influence on educational achievement, and that dual language learning enhances general linguistic skills. (RW)