@SeasonFinale
It's the sixth form of a state comp with approx 270 students in Y13.
The school is non selective and sixth form offers alternative study courses in addition to A Levels so the proportion of potential Durham calibre candidates is reduced in comparison to say the sixth form of a more selective secondary school/independent or grammar school.
Location wise it is decidedly "North of Watford Gap" which possibly lessens the appeal of heading off to a Uni such as Durham, Exeter, Bath etc, where 30-40% of students may be from a private school backgrounds, for many pupils/their parents.
The charts of A Level grades achieved by the school's Y13s in previous years show that pupils taking non STEM A Level subjects do not achieve as highly as pupils do in STEM subjects. This might also account for a lower proportion likely to achieve the grades usually required by Durham as a lot of the STEM high achievers seem to want a career in medicine which rules Durham out.
To many pupils, heading off to Uni is exciting but daunting in many ways. Choosing a Uni where most people are coming from potentially similar backgrounds possibly offers some reassurance. On the other hand, a friend's niece had spent her entire school career in very hard to get in to private schools in the home counties and refused to consider Bristol as an option, despite it's good reputation for her chosen course. She said she was dying to get away from that culture (many from her super selective secondary school sixth form headed off to Bristol each year apparently although I'm not sure it features as having a large proportion of privately educated applicants).