Unlike in elementary schools, high school teacher effects may be confounded with unobserved track-level treatments (such as the AVID program) that are correlated with individual teachers. I present a strategy that exploits detailed course-taking information to credibly estimate the effects of 9th grade Algebra and English teachers on test scores. I document substantial bias due to track-specific treatments and I show that traditional tests for the existence of teacher effects are flawed. After accounting for bias, I find sizable algebra teacher effects and little evidence of English teacher effects. I find little evidence of teacher spillovers across subjects.