Honestly? I'd remortgage, downsize or, to be absolutely honest, not have put them in in the first place if it wasn't absolutely comfortable. It's the same reason I bought a house I could afford, not just within my price range. Because life isn't predictable.
The modern day middle class seem to have the idea that private school should be accessible necause you can scrape over the line. Ignoring any risks of ill health or job losses that might occur and result in the same problem.
Fee changes are not the sort of thing that bother parents woth kids at Eton.
It's a fable, and frankly poor planning, to have thought it was ever achievable or a safe bet that said child would have a pernament space if parents are that close to the line.