Hi OP, both of my girls went there (they're now 20 and 19) so feel free to message any questions.
Basically it's a great school for girls to learn without "boy" pressure (although KEGS, the boys equivalent, is down the road so they do have joint discos etc and both of mine went there for sixth form as they take girls then, so they're not completely cut off!).
It is fast paced learning and can be competitive (a B grade can be the end of the world) but the girls spur each other on to do well and tbh the types of girls that go there will put themselves under pressure naturally.
There can be pressures with self harming etc but I think you find that anywhere these days.
They have lots of opportunities for extra curricular activities and I'd encourage your DD to do those from the start before year 9 when there's a heavier workload, but again I think the types of personalities that go there cope with lots of work and hobbies.
You'll find lots of girls know nobody there...eldest DD was the only one from her school but soon made friends...the bus commute is great for this, and DD2 only knew her sister but again soon made friends.
The day can be long...mine got the bus at 7.30 and got home at 5 so much longer than local friends and they then have homework so social life in the week was non existent, especially as friends from CCHS are scattered...which also means be prepared for lots of taxi-ing for parties and weekends!
Both left with all 9s in GCSEs and are now at uni.
It has good and bad points of course but if your DD is bright and wants to do well and get stuck in she'll love it.