This research used prospective longitudinal data to examine the associations between first-grade teachers' over- and underestimation of their students' math abilities, basic reading abilities, and language skills and the students' high school academic performance, with special attention to the subject area and moderating effects of student demographic characteristics. Teachers' inaccurate expectations in first grade predicted students' math, reading comprehension, vocabulary knowledge, and verbal reasoning standardized test scores at age 15. Significant interactions between students' family income and teachers' misperceptions of students' math and language skills were found, such that teachers' over- and underestimation of abilities had a stronger impact on students from lower income families than on students from more affluent homes. In contrast, the effects of teachers' misperceptions of students' basic reading abilities on performance at age 15 did not differ by income. These results have implications for understanding the complexities of self-fulfilling prophecies and for understanding the achievement gap between students from disadvantaged and advantaged homes. (Contains 6 tables, 1 figure and 3 footnotes.)