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.