@StarlingsForever also mentioned that he/she (I'm going to go with "she") is very much a higher earner, earning much more than me (for example). And I'm not strictly a "marginal" private school parent; we're just about able to pay up and grumble if we choose to.
Maybe her professional and social circles don't include many people who are "marginal" families. Precisely because I have some social awareness, I know there are many tens of thousands of families on (say) £75-150k (1) for whom this tax won't be manageable given marginal tax rates of >62pc plus cliff-edges (2) others who theoretically "could" manage it by making further sacrifices but will choose not to; and (3) others who will push themselves even harder and manage with great difficulty.
In all those cases I'm saying it's not "fair" given how much tax these people already pay, contributing ~£30k to the Exchequer per child in school; in cases (1) and (2) I'm saying it greatly undermines the fiscal argument; I think we're all agreed that nobody really knows how many families fall into each group, but the risks if Labour are wrong are obviously very large.
Additionally" because I have social awareness", partly through my workplace where I employ around 200 people on lower wages (and before somebody pipes up, I'm fond of them and I'm an excellent boss, they tell me), I know they don't really care about or hear about this policy, but they only quite like the idea of making state schools better. If the economics don't work, they're being sold a pup, and that's why the economics are front and centre of this debate, where Labour have put them.
We've heard her tell us she's done her school fees and has heaps of spare money. Perhaps we file this under "luxury beliefs".