don't worry nonconformist, ask away.
i'm not sure how much they challenge the very bright. Although my dd is very able, she's still within the range of everybody else (if that makes sense?) They (obviously) match the reading books to the level of the child and the type of maths worksheets etc they do. But with small classes and 2 members of staff they can individualise the work more. And you tend to find that the teachers are hugely approachable and more than happy to chat in the morning or after school so if ever you think the work isn't challenging enough you can always ask them why? In a school like psm where every student is important in terms of fee income (and in other ways too!) the school is always willing to take on board individual parent's requests (within reason).
They do loads of art, school plays, music, languages, lunchtime clubs ranging from playing chess with the head to playing board games with some teachers so there are always options for different interests. I suppose a lot of the challenges come from trying different things. If you want your daughter challenged but not pushed too hard then maybe it's the extra curricular type stuff that will appeal? And also, because the infants department has the equivalent number of kids as a state school reception class they often do things with the three classes joined together (so the reception kids get lines in the nativity etc, they all join together for ballet, the older ones get to be monitors and head up teams on sports day) and you often find that the younger ones love being with the older ones and the older ones develop a strong sense of responsibility in looking after the tiny kids.
I think those that plan on leaving at 11 probably get outside tutoring but you're right, probably best not to mention that at all, they're kind of sensitive about it!
I'm not sure yet what i'l1 do at 11. A lot of it will depend on what dd wants and the kind of teenager she looks as though she'll become. i know that a few years ago i went through a very messy divorce and the level of support both she and i got from the school was incredible. These are the things you don't forget and i like the thought of her spending her teenage years surrounded by teachers who will notice if she's going off the rails! She might not like it quite so much though!
the all girls thing is what is, only you can decide if it's enough of a reason to walk away from one particular school. I always planned to get her involved in activities outside of school so she'd mix with boys. It hasn't worked, she's at the age where they all prefer to be with their own gender. and really, that's probably what happens at school anyway so by sending her to a coed all that's happening is the pool of potential mates is halved because they won't go near the boys. Maybe?
Really i think you'll know when you visit a school if it's the right one for your child. psm just felt right to me the very first time i walked in the doors in a way that none of the others did. And all the kids are happy there. The best way to really judge is to go and see it when the girls are there. I don't know if they arrange ad hoc visits or if you have to go during an open day?