The COVID-19 pandemic has led to widespread school shutdowns, with many continuing distance education via online-learning platforms. We here estimate the causal effects of online education on student exam performance using administrative data from Chinese Middle Schools. Taking a difference-in-differences approach, we find that receiving online education during the COVID-19 lockdown improved student academic results by 0.22 of a standard deviation, relative to pupils without learning support from their school. Not all online education was equal: students who were given recorded online lessons from external higher-quality teachers had higher exam scores than those whose lessons were recorded by teachers from their own school. The educational benefits of distance learning were the same for rural and urban students, but the exam performance of students who used a computer for online education was better than those who used a smartphone. Last, while everyone except the very-best students performed better with online learning, it was low achievers who benefited from teacher quality.