You always get knobheads, no matter the intake. If 3% of people are knobheads, with a class of 30 there's a 60% chance of having at least one knobhead. With a class of 20 it's 46%, with a class of 15 it's 37%.
You seem to be assuming that admissions is based on a lottery and expulsion cannot occur.
You might note that at what are considered "good schools", not only are neither of those things true, but trying to implement either would cause the school to be quickly abandoned.
You might note that at what are considered "good schools", not only are neither of those things true, but trying to implement either would cause the school to be quickly abandoned.