This article provides the first causal evidence of a system-wide corequisite reform in Tennessee, which mainstreams underprepared students into college-level courses with concurrent support. Using regression discontinuity and difference-in-regression-discontinuity designs, we find that, for those on the margin of college level, students placed into corequisite remediation were up to 18 percentage points more likely to pass gateway courses by Year 1, compared with peers placed into prerequisite remediation, and they were 10 percentage points more likely to pass subsequent math than peers directly placed into the college level. The positive effects in math gateway completion were largely driven by efforts to guide students to take coursework aligned with the requirements for their program. We do not find significant impacts on long-term outcomes.