Weeding out as I've seen it is a class that requires a certain level of commitment and ability to either plan your work or tough it out that a high school just can't really prepare anyone for. So in a way the student isn't a "weed" but their motivation or maturity might be and they're free to retake the class once they know that university will require them to put in more work than high school. If they can't put in the work then completing a thesis and graduating is going to be very hard and that happens the last year of uni so better to set the expectations early with a "weed out" class.