Or to put it another way, since Labour are promising 6500 extra teachers from this money
There are currently 468,000 state school teachers in the UK for 8,890,000 students. That's 19 students per teacher.
If you had 2.4% fewer students (the insignificant, marginal amount of private school kids who you suggest will just fill the spaces of falling student numbers) then to have the same student/teacher ratio, you would need 11,232 fewer teachers.
So if you leave it all alone, let the student numbers drop instead of replacing them with private school kids, you've effectively gained 11,232 teachers. And these are real, existing teachers the schools actually have - not teaching vacancies they can't fill.
If on the other hand you bring in those 2.5% private school kids from the reduced VAT take, you'll get the money to pay for 60% of the 6500 teachers promised, say 4000 extra teachers.
4000 extra teaching vacancies with the policy versus 11,232 extra existing teachers without the policy.
Everyone is worse off from this policy. Not only private school kids but state school kids too.