Unlike in elementary school, high-school teacher effects may be confounded with both selection to tracks and unobserved track-level treatments. I document sizable confounding track effects, and show that traditional tests for the existence of teacher effects are likely biased. After accounting for these biases, high-school algebra and English teachers have much smaller test-score effects than found in previous studies. Moreover, unlike in elementary school, value-added estimates are weak predictors of teachers' future performance. Results indicate that either (a) teachers are less influential in high school than in elementary school, or (b) test scores are a poor metric to measure teacher quality at the high-school level. Appended are: (1) Distribution of number of teachers in each school-track year cell, (2) Distribution of student observations by the distribution of number of teachers in each school-track cell with the implied required degree of freedom adjustment; (3) Distribution of the number of classes per teacher and per teacher year; (4) Dispersion of teacher effects (estimated without track fixed effects) across and within tracks. Also included are: Appendix Note 1: Matching Teachers to Students; Appendix Note 2: Deriving the Degree of Freedom Adjustment for the Size of Tracks; Appendix Note 3: Is the Lack of an Effect for English Teachers Due to Measurement Error?; Appendix Note 4: Estimating Efficient Teacher Fixed Effects; and Appendix Note 5: Evidence of Student Sorting to Teachers.