However, it does mean that some children who would NOT be outliers in a mixed ability system become outliers in a selective one. Because selection is usually based on a single test on a single day, false negatives and false positives are both possible. That means that a low-middle ability child can end up being seen as 'exceptionally low ability' in a selective school (and may end up as isolated as a child with significant SEN would in a mixed ability school, even though their ability is common in the general population). Equally, higher middle ability children may not pass on the day, and become 'high ability outliers' within a non-academic school.
I get what you are saying Teacher. It's like what Bertrand seems to be saying. While it makes sense at first. I think you are wrong about this.
IQ falls into a classic bell curve shape. True outliers have to be 2 standard deviations from the mean. That means the most able outliers are the top 2.5% of the population. Selective schools are taking more like the top 10%. So, if comprehensive schools were denuded of all of the top 10% (they won't be for a myriad of reasons, but lets just assume) then the kids in the left in the comprehensive schools would all be in the buldge of the bell curve. This means even those on the verge of being in the top 10% would have a reasonable number of peers around, simply because they are in the buldge, not the ever thinning tail.
Disclaimer: my stats learning is over 20 years old and hasn't been used since, so I am prepared to be corrected.
So the problem of those on the edge of being selected for grammar schools is one of disappointment and frustration, not a lack of peers.