I think we need a 5 years timeframe: long enough that most people have actively chosen school with VAT rather than stayed trapped. This reflects the long term viability of the policy.
At that point, consider how many children are in private school rather than state school. Eg I'd expect it to remain stable at 7%, as it has for years. If the proportion in private school has dropped from 7% to 6.3%, that means 10% have switched. Which is the tipping point at which the policy costs taxpayers money instead of raising it.
IF in 5 years time
- 6.8% of children are still being educated at their parents expense instead of paid for out of taxes, to match the government estimate of 3% migration
- And there are significantly more teachers in state schools than there are now (won't hold them to 6500, but at least 4000)
- And at least 75% of state schools have free breakfast clubs
THEN I will very happily admit that I was mistaken, Labour are geniuses, and all is right with the world.
IF on the other hand, private school numbers have dropped by 10% and only 6.3% of children are still educated at their parents expense - with the state picking up the education costs for the extra 0.7%...
...or if it's a slightly smaller fall but with more expensive SEN included (say 6.5% still educated privately, but only 18 % of private students now having SEN, indicating double the migration rate for students with SEN)
Then I will laugh bitterly, and continue to say that Labour are idiots and so are the people who have increased their own tax costs - and reduced what the government can provide - out of misguided ideology, envy and an inability to do maths.