You've hit the nail on the head. Census day has already happened. The timing of this tax is such that extra pupils that flow in to the state system from January all the way to the next census day in autumn 2025 (and beyond, as it takes time for money to trickle through), will be completely unfunded for most councils. But the schools will need to absorb them anyway and make it work.
Plus, VAT is such that any take goes into one big government pot and it is NOT ring fenced, as is often mistakenly noted. There will be no accountability with this particular tax as to spend and distribution, as there cannot be - such a mechanism does not exist, so we as taxpayers will simply never see measurable impact. We will never know how much was actually made, or how much made its way into the education system and if it actually funded teachers/breakfast clubs/etc etc.
There will be a lot more teachers leaving as a result of this (far, far more than the mythical 6000 odd teachers who haven't even been trained and hired yet as part of this "plan"). Who wants to teach under these increasingly shit conditions. That in turn will leave larger classes untaught in squashed classrooms around the country.
It is just bonkers that the government thinks this is a good idea, as just puts more pressure on an already shaky system. It will be even worse if many independent schools close at short notice (which is the end goal for Labour) as then there really is no alternative for families other than state, which will have neither the funds, teachers or space to educate all to a high standard. This disproportionately affects current senior school children. Primary schools will likely be fine long term with spaces, but it's really really unfair on older children who are already struggling with lack of teachers and poorer GCSE and A level outcomes in the short term.