In traditional schooling, time is a constant and understanding is a variable. A fifth-grade class will spend a set number of days on prime factorization and then move on to study greatest common factors — whether or not every student is ready. If student turns in shoddy work in a ‘flipped mastery’ class, she can’t move on to the next level. But there is another way to look at schooling — through the lens of a method called “mastery learning,” in whi...