Schools need to ask themselves why they have such a high entrance criteria. Is it for them - or the pupils?
Um- it's the fault of parents actually! You know- the ones who only send their precious children to sixth forma that have exceedingly high pass rates! 
My 6th form has been struggling with numbers for years, for a variety of reasons.
We're in an area with "league topping" super-selective grammar schools, that always feature in top twenty schools nationally (GCSE and A Level). Our really high flyers leave us after GCSE to go to these 6th forms, so our average ability intake at 6th form is fairly middling. Our performance at a level is also fairly middling. We don't have poor value added, we don't have amazing value added. We allow anyone getting E or above at end of Y12 to continue the subject in Y13, so in things like physics or maths we do have some Es and Us at A level, mainly because those students shouldn't really have been doing those incredibly hard subjects, but they really want to (perhaps for next level of study). We also have students that because of their ability (well, technically because of their prior performance at GCSE) have A level targets of D (and even an E this year). Even if these students get above target, they're still middling results (Cs and Ds) and parents look and the headline pass rates and refuse to consider us for A Level.
What are we supposed to do? Keep taking mediocre students, so the passrates stay mediocre? Our GCSE are truly outstanding! But Y11 pupils know that if they go to one of the grammar schools for 6th form they'll get stellar a level results too (because if they don't, they'll be out after Y12!! We get one or two each year return to do Y12 again, or Y13 with us).
Our students that stay are v happy, because our 6th form is a lovely place to be, where the staff do actually care that the students are happy, and enjoy all the courses they've chosen (even if they weren't necessarily the "correct" choices for those pupils), we do not have many that drop out of 6th form (and certainly only force maybe 1, perhaps 2 students to drop an a level per year, where they really will not get a grade, and could do something else more beneficial to them in their time, e.g. an AS level in Y13, work experience, EPQ to get them more UCAS points etc.). But we cannot stop the spiral that parents see national average a level results from a school way above national at GCSE, and they won't consider us.
Maybe that will change now the dfe performance tables present KS5 performance differently and in terms of progress too? But I doubt it. Parents only look at percentage A-A, A-B. It doesn't matter if your pupils are actually only "capable" of a D and getting C's and Bs!