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.....
I don't want to jump into this sub-thread, but I will anyway because I find it interesting.
The one aspect not being mentioned in the discussion of pass/provisional/fail is time frame and opportunities to practice/develop. Given ample time, most students will eventually pass a course, they just might not pass it outright in the first go-round within the time frame of the original schedule. Approximately 30-40% of my cave students do not pass outright on the first attempt and usually need some additional development time. This is where the "provisional" idea comes into play.
However, every now and then I get someone that does outright fail, usually it's when there is a concern about safety. I think John's story about the cavern instructor that was panicky during a C1 class is a great example of a person that should have failed due to not being mentally/emotionally mature enough to handle technical diving. Not too long ago I had someone show up for a beginning cave class that was very clearly lacking any form of cardiovascular fitness (he was huffing and puffing while walking without gear on), I sent him home after the first field day and told him he needed to work with a medical doctor and get his health in order before he could take a cave class with me. The bottom line here is that diving and tech/cave diving add additional risks and at some point we need to draw a line in the sand and say "sorry, this is not for you."