What is a Malthusian Drill?

A Malthusian Drill is an organized form of excersice. We can infer it's similar to the excersice sessions in the military.

From behind a door in the corridor leading to the Beta-Minus geography room, a ringing soprano voice called, "One, two, three, four," and then, with a weary impatience, "As you were."

"Malthusian Drill," explained the Head Mistress. "Most of our girls are freemartins, of course. I'm a freemartin myself." She smiled at Bernard. "But we have about eight hundred unsterilized ones who need constant drilling."