SCHENECTADY, N.Y.—A principal-turned-consultant has built a movement—and a business—on overturning how teachers have graded for generations. His alternative: “grading for equity.” Joe Feldman preaches that students should be able to retake tests and redo assignments. There should be no penalties for late work and no grades for homework. No points for good behavior, classroom participation or perfect attendance, either. “When you include those in a grade, you’re bringing your implicit bias into the grade because not all students learn in that particular way,” Feldman told dozens of teachers gathered for a training session in Schenectady, N.Y., one Wednesday afternoon in...