"The number of classes must be less than or equal to the number of classes considered."

Where: Settings › School › Regulars (owner).

Why: a student is a "regular" when they came at least N times in the last M sessions of a class. N (1 to 20) can't be larger than M (1 to 50). Default: 2 times in the last 6 sessions.

What to do: lower N or raise M so that N is less than or equal to M, then save.