Hello! Just seen your post.
My dd went to Wheathampstead House, the primary of STAHS, before moving to a different private girls school for secondary. They obviously have lots of friends who went on to the High School though, and I have lots of friends who have dds there.
Before deciding, I would advise you to look carefully at the A level results for STAHS this coming summer. Last year's A level results there were very disappointing, and loads of girls missed their university offers and had to go to their insurance or go through clearing.
Basically, STAHS handled the transition from old modular A levels (with AS levels in Year 12) to new, Linear A levels very badly. The headteacher decided to have no proper exams in Year 12, and advised girls to take the 'year off' to "take up a new hobby, get a job, fall in love". When they came to Year 13, the yeargroup hugely underachieved. I'm so glad my daughter changed schools.
This year therefore, they've cracked down on the current year 13 and year 12 - no fun whatsoever, all focused on exams.
So I would look and see what their results are like this year - if they've improved, then that's good, but I worry the school has been so burned by its bad experience that it will feel compelled to load girls with tonnes of homework and holiday work throughout the two years of sixth form.
STAHS cannot afford to have another year like this year.