As someone who grew up in Cornwall, I would not say - 'stay clear of Cornwall'.
Cornwall has 11 - 16 comprehensive schools and large sixth form colleges.
I - and my sisters - did exceptionally well in both our 11 - 16 school and our sixth from colleges. It was the days before A* and I think we got a total of 35 A grades at GCSE between us and then all As at A-level.
My youngest sister did her A-levels at Kevics (parents had moved to Devon by then) and got all As at A-level. She did not want to go to a girls grammar in Torquay, plus the girls grammar didn't offer further maths A-level, which is what she hoped to do. In her Physics A-level she scored one of highest scores and was invited to a special 'do' in London.
Two of us went on to Cambridge and one to Durham. We are now a hospital consultant, chartered engineer and social worker.
I have just holidayed in the area we grew up in and bumped into lots of old school people. They, too, were success stories and included city lawyers, GPs, senior teachers etc.
My old 11 - 16 school claims it is now the best in the south west and - having looked it up - it has great results. A number of pupils were getting over 12 As or A*s at GCSE and its overall 5A-Cs including English and Maths were approx. 70%. (Remember that all Cornish schools are non selective and that in each school there will be children of all abilities, so not everyone gets 15As at GCSE).
Torquay and Plymouth have grammar schools which mean that the whole issue of secondary education is more pressured. (And a lot of grammar schools, having selected their pupils, seem to rest on their laurels a bit). Kevics had a great hippy atmoshphere when my sister was there.