High school GPAs (HSGPAs) are often perceived to represent inconsistent levels of readiness for college across high schools, while test scores (e.g., ACT scores) are seen as comparable. This study tests those assumptions, examining variation across high schools of both HSGPAs and ACT scores as measures of academic readiness for college. We find students with the same HSGPA or the same ACT score graduate at very different rates based on which high school they attended. Yet, the relationship of HSGPAs with college graduation is strong and consistent, and larger than school effects. In contrast, the relationship of ACT scores with college graduation is weak, smaller than high school effects, and the slope of the relationship varies by high school. [This paper was published in "Educational Researcher" v49 n3 p198-211 2020 (EJ1250589).]