Bubbles in my school, entry criteria are set by the head of sixth form, not subject experts - the maths department was overruled when it came to setting entry requirements to maths A-level.
At the same time, it is impossible to tell beforehand from GCSE results, even if the student meets the entry requirements, who will get a U and who will not, because it depends on how hard they work. I've seen kids with lower grades at GCSE go on to get good grades at A-level, and a kid with an A* at GCSE get a U at AS. You cannot refuse entry to an A-level course if they meet the entry requirements, and you cannot include 'we won't let you on if you look like a slacker' as an entry requirement.
Thus, there will always be students getting Us at AS, regardless of how highly their teachers are paid, because at the end of the day it's not the teacher sitting the exam.