Jonathan Zimmerman teaches history at New York University and lives in Narberth. He is the author of "Small Wonder: The Little Red Schoolhouse in History and Memory" (Yale University Press). He can be reached at jlzimm@aol.com.

The Cherry Hill Board of Education and its teachers recently agreed on a new contract that extends the school day by 30 minutes. Over the course of a 180-day school year, that comes out to about 14 more days of class.

But it probably won't make much of a difference, at least not for high school students. That's because the board tacked the additional time onto the beginning of the day, forcing high school kids to show up for school at 7:30 a.m. instead of 8.

They won't be awake. Sure, they'll trudge into class and go through the motions. But as a growing body of research reveals, adolescents' bodies - and minds - don't really rouse that early.

Consider a recent study of Chicago high school students. It found that they got lower grades in their first-period courses than in the rest. Standardized tests also showed that they scored worse in subjects taught at the start of the day.

At the Air Force Academy, meanwhile, first-year cadets who began class before 8 a.m. performed substantially worse in all of their courses, not just the earliest ones....