Fees at DD's school range from about 6k per year at age 3-4, to 12k per year in the sixth form.
Their turnover last time I looked at charities commission website was £12m; their surplus was 300k iirc.
The school is non-profit-making so any surplus is carried forward to be used for the benefit of the school in the future. Lots of improvements going on at the moment re dining hall, new science block planned, expansion of junior so not at all worried about surplus.
We have a fantastic bursar who keeps a good holdo n the purse strings.