This is what I find obnoxious. Who decides how much "impact" is enough? Surely that is up to the earner herself.
If OP has arranged her life and finances with no reliance upon anyone else to afford five luxury holidays a year, or to fund her kids, or whatever, who is anyone else to say that her choices are "more than enough" and that she can "afford" to pay toward this unrelated child's school fees?
It's not necessarily a "household pot of money" just because some outsider decrees it to be so. I have never mingled finances with anyone I've lived with, and never will. We pay our share of agreed-upon bills and the rest is private.
The idea that my money could be claimed by a third party that I have no relationship with (the school) to benefit someone I have zero kinship with, or legal duty toward, is just grim. That so many commenters here think it's A-OK is concerning.
We have legal marriage to distinguish between those who are obligated to one another financially, and those who are not. Everyone else is basically just roommates, whatever they may do behind closed doors.
This is reason #347 to never get involved with someone who has children, as far as I'm concerned.
(again, I realize the bursary office can do as it pleases but it's wrong in principle to go after a bystander like the OP).