Even nowadays there are plenty of students who struggle with the transition to A level and don’t do as well as they could at AS, but pull it round before A level.
One of my dc got BBCD at AS, but by a concerted effort in Y13 got A*AA at A level. Luckily their 6th form had a floor of DDD for progression into y13 - they didn’t automatically chuck out dc who got below that, but did bring them in for a serious chat about how to move forward, including the option of retaking y12.
I agree with tiggytape - schools select the students after GCSE, and that should constitute a commitment to see them through to A level, unless the student really duffs up their side of the deal by not showing up to lessons or doing no work at all.