Yes. We were told this. I was at a competitive entry school and we were told we were all top 10%.
me too, we had to pass the usual exam but also do a full day of brain teaser type tests at the school to get in. my school was was top 3 in the country while i was there and on my first full day there we were told how many children had applied for each place and that we were very unusual & privileged to have got in. It was a total surprise because my mum was always calling me stupid and I didn't know just how far up the league tables my school was until I got there.
I had very low self esteem because I internalised this idea that all the girls at my school were bright and capable, except me. I thought I was uniquely crap and didn't like participating in anything school related as a result. Looking back, I can see that many girls felt like I did. I remember a friend telling me sadly that she wished she was at another school because she had only got in via the waiting list and wasn't smart enough to be there. Another girl told me "what's the point in being smart anyway.. there are loads of smart people. I lack all the other important skills like interpersonal skills." (she later had a breakdown and dropped out of her Cambridge degree).
I can remember a teacher in my GCSE year telling me that "you are so privileged to be here but a place at this school is wasted on you" (tbf I was quite lazy but I still did my homework, didnt mess around in class, and was an A grade student in his subject). Despite all my GCSE & AS level grades being As or A*s it was made clear to me that the school wouldn't support an Oxbridge application as they only wanted to send their "best and brightest" there for interview as the school was supposedly so full of amazing kids, of which I was not one.
For other girls who perhaps had less critical families and were pretty, sporty, arty or whatever else I can imagine that they didn't come away with the same feelings of self loathing that I did. I have always felt like there were 2 tiers of public school children- the outwardly confident, glossy ones with charisma who thrived. And the ones who felt they were unworthy and inadequate in that environment.