We study teachers' choices about how to allocate class time across different instructional activities, for example, lecturing, open discussion, or individual practice. Our data come from secondary schools in England, specifically classes preceding GCSE exams. Students score higher in math when their teacher devotes more class time to individual practice and assessment. In contrast, students score higher in English if there is more discussion and work with classmates. Class time allocation predicts test scores separate from the quality of the teacher's instruction during the activities. These results suggest opportunities to improve student achievement without changes in teachers' skills.