When we first enrolled our son in Japanese school, there were occasions when he came home earlier than I'd expected. The first time, I happened to be at home. "Why were you dismissed early?" I asked my son. "I don't know," he shrugged. "The teacher said something, but I didn't understand."

The second time it happened, I came back from an errand to find my son locked out and upset. I called another mother to find out what was going on. "There was kenkyu jugyo today," she explained. "That's an experimental lesson conducted with one class of students and observed by all the other teachers in the school. The other students are sent home early so their teachers are free to observe the lesson. It's always marked on the school calendar so parents get home early."

Kenkyu jugyo is rendered into English as "research lesson" or "lesson study." Japanese parents know to check for such days because experimental lessons are conducted at virtually every elementary and middle school in Japan, and are the basis of konai kenshu (in-school research), a continual process of lesson improvement and school-based professional development for teachers.