Thanks, everyone, for your input. Alison - I read that blog just before I posted on here! Deb, Jenny - your views mostly echo what I've believed intuitively but wanted to see if the stats actually bore that out - our deputy head is giving us the opportunity to pitch for stronger entry requirements to try to improve results but is most likely going to be sold with some convincing numbers.
I've started doing some (admittedly very basic) analysis of data and so far there is no clear cut pattern emerging! Students with strong Science grades (esp. Biology) generally tend to do well... except I've got a small group of high-flying y13 students who are all aiming at A/A* for me but got C's in Biology, though they all also have high grades in History/Geography/English instead. I read on a related blog that students with strong Humanities skills may struggle in year 1 but get better in year 2 as the demand for synopticity and extended lines of argument gets stronger so there might be something in that (but, again, not across all students). Does seem that those who didn't do triple science at GCSE (a relative minority at our school, but possibly proportionally over-represented in our Psyc cohort) tend to be our weakest students. Maths isn't a particularly useful indicator at my school as nearly all of them get A/A* at GCSE but this doesn't necessarily reflect any natural mathematical ability! High English Lang grades seem to be reasonably indicative but again not conclusively as a few strong students with weak English
grades. So not quite sure what I'm going to take to my deputy head yet!