I think it would be rare for an entire class to score ultra high or ultra low on a test, or more importantly, over the course of an entire semester. And if it started to steer that way, teachers could ‘calibrate’ the difficulty level. Like any competitive event, people tend to spread out.
By my point is, if everyone in a class is learning the material (I prefer to drop the case of no one learning it, because then it might be an issue of whether it is the right material for that group at that time or maybe it’s a teacher problem)...
If everyone in a class is doing well, do you HAVE to have some D’s and C’s?
If I remember correctly, in my high school geometry class in the 9th grade, almost everyone in the class “got it”. Should those who were 1 or 2 points behind have been given a low grade?