The public narrative surrounding efforts to improve low-performing K-12 schools in the U.S. has been notably gloomy. Observers argue that either nothing works or we don't know what works. At the same time, the federal government is asking localities to implement evidence-based interventions. But what is known empirically about whether school improvement works, how long it takes, which policies are most effective, and which contexts respond best to intervention? We meta-analyze 141 estimates from 67 studies of turnaround policies implemented post-NCLB. On average, these policies have had a moderate positive effect on math but no effect on ELA achievement as measured by high-stakes exams. We find evidence of positive impacts on low-stakes exams in STEM and humanities subjects and no evidence of harm on non-test outcomes. Some elements of reform, namely extended learning time and teacher replacements, predict greater effects. Contexts serving majority-Latinx populations have seen the largest improvements. [Empower Schools and the University of Virginia provided financial support.]