Absences in Chicago Public High Schools are 4-7 days per year higher in first period than at other times of the day. This study exploits this empirical regularity and the essentially random variation between students in the ordering of classes over the day to measure how the returns to classroom learning vary by course subject, and how much attendance in one class spills over into learning in other subjects. We find that having a class in first period significantly reduces grades in that course but does not affect grades in related subjects. We also find that having math in first period reduces test scores in all subjects and reduces grades in future math classes. These effects are particularly large for black students. For classes other than math, we find little effect of having the class in first period on test scores or long-term grades.