We dont live in a "theoretical world". But you are still "failing" people by telling them at the door to go home, and my "bell curve" still exists, its just that you fail them before they start, not at the end of class.
In essences, by keeping those you believe to be "failures" out, you would be manipulating the outcome to fit your preconceived bias.
And I admit I do not share you abilities and knowledge of education theory, but I have taken enough calculus and statistics classes to know that, at the end of the day, the math doesn't "lie" there will always be data points at either end of the curve. As you stated in your first sentence, "educational theory", not educational fact.....