Hi Nervous Nelly,
The Devon forum is ever so quite!
I don't know the schools you are interested in so I can only give you general ideas.
Have you looked at the Ofsted reports for both schools? If you haven't already done so, read them from top to bottom. They may (or may not) confirm your impressions of the schools.
There is also the 'Good Schools Guide'. Devonport High School for Girls has an entry in it. The book is expensive but many large libraries hold a copy.
Otherwise there is of course the league tables. The BBC website is a good source. It allows you to sort the results by GCSE pionts, Value Added or A level results.
If you have any question, go back to the school and ask. Most schools are quite happy to help. It is also an opportunity to have a second look at the school.
You can also find out how many hours is spend weekly per subject. Some schools spend more time on core subjects, others have more subjects with less time on each.
Schools have different policies for the 6th form admision.
Here's a quote from an interesting post:
Whilst I think it's ok to compare schools in terms of their GCSE performance, I think it's completely meaningless to use A levels as a fair comparision of school performance for the following reasons.
Most Grammars have very strict rules regarding GCSE targets that pupils have to achieve to stay on into the 6th Form. Usually this means if you don't get A grades in the subjects you want to study at AS/A level then you haven't a chance of staying on. 4 grade As at GCSE is normally the minimum requirement, 6 grade As is not uncommon.
Having culled year 11 - they then cream-off the best performing pupils form the Comprehensive or Independent Schools.
It's not usual for the top Grammars to demand at least 6 A*s at GCSE for external pupils who want to come in to the 6th form. There is no shortage of takers.
Most Grammar heads are very smart and know that league table performance is the main measure, but also that within the DfES ratings, A level performance gets a much higher weighting - hence the cull.
Don't get me wrong - I'm not saying that the quality of teaching in Grammars isn't very good or excellent - but if you have weeded out all of your poorer performing pupils at GCSE and replaced them with outstanding pupils, from external Comprehensives or Independent Schools, then you are bound to get great results.
GCSE results are a much better measure of performance based on the original raw intake at year 7.
Strangely enough, a pupil from a moderate Comprehensive who gets 6+ A*s might actually do better to stay on at the Comprehensive. Bright kids who get excellent GCSEs and A levels from a Comprehensive will probably get much better offers from the top University's than pupils from top Grammars where 80%+ get grade As at A level.
And finally, a link to a thread about a similar quest:
This one could also be of interest:
starting with Tone's post
Hope this helps