I’d say again, that just because you, as a a clever person, or kids you know who are clever went to college, doesn’t mean the statement isn’t true.
On a broad scale across the whole country, I would say it is true and that those who are higher achievers at GCSE are more likely to be found in school than college. This won’t apply to all areas because some don’t really have sixth forms in schools and provision differs a lot. Given 20% of sixth formers are in private education and almost all of that is in schools, straight off, you’ve got a good chunk in schools. Of course lots of high achievers go to college too. Some prefer the idea of college, some find more choice of subject at college and in some areas it’s college or nothing. But where there are sixth forms in schools, those staying tend to be more academic. It’s because schools often focus wholly or mostly on A Levels. Those doing vocational qualifications, who broadly across the whole cohort tend to have lower GCSE grades, tend to go to college. And again, I’m not saying clever students never do vocational qualifications or those who do them don’t go onto be hugely successful.
So, given vocational qualifications tend to be college based, most of those with a lower GCSE profile go there. There are far more options. In itself, this means there is an imbalance in terms of where those with higher and lower prior attainment end up. Those with higher attainment are more spread between schools and colleges. Those with lower attainment are more concentrated.
Earlier in this thread I put in a link which showed some evidence of prior attainment and where students studied. It is true that schools have a higher or operation of their students with higher attainment, but this doesn’t mean everyone who goes to college has lower attainment or doesn’t do well. Of course students with all grade 9s go to college and some get all A* at A Levels and go onto Oxbridge etc. Look too, though at where those at Oxbridge and RG unis did their sixth form educations. Again, both sixth form colleges and schools send students, but lots are coming from the more traditional school route.