We examined the results of a large-scale national survey of online secondary education in China. The online survey of 33,194 high school students and 5,667 teachers has provided comprehensive and representative data regarding the quality of online education and its implementation during the pandemic. Questionnaire surveys administered to students and teachers in different grades and comparative analysis of different cohorts revealed group heterogeneity of the online learning effect. The findings have elucidated the emergency teaching situation in China during the pandemic, yielding targeted suggestions for school education following the resumption of classes.