Thanks @Icouldbevioletsky 😊
Ah, that's interesting about the CAF always ending at the end of the summer term! You're right, that does make it a fair comparison. Although I suspect that slightly more families may hold onto their state places this year than previous years due to uncertainty: more private schools closing, and also the general uncertainty about what Labour will tax next (I certainly feel future tax raids hanging over my head, and feel less financial confidence as a result)
Good point that the birth rate difference probably isn't significant.
I do wonder what proportion of children who go to private school apply for state as a backup? That's probably the most significant uncertainty.
To me it seems a no-brainer to do that! But if the CAF numbers of those who applied to state but then go private averaged 3-3.6% in your area but about 6-7% of children go to private school (nationally and across all ages) we could make a very, very rough (strongly caveated) guess that about half of those who will end up in private apply to state as a backup. (I'd expect it to vary hugely by area, but it gives us something to work with).
That would suggest overall movement from private to state in your area at Year 7 of about 10%, as a very, very rough guess.
Given that Y7-Y11 is the stage that many parents consider the most impactfull,, you'd expect more change in prep and 6th form than Y7 entry. So overall change in say 5 years time would be higher than the Y7 entry change.
So whilst we certainly shouldn't draw too much from it, I'd say it does support an expectation that the policy is likely to be loss-making.