I have namechanged to give a more full answer here to OPs original question.
We are putting 4 through private secondary, although due to age gap maximum strain is 3 at once for 4 years. These are London day schools where fees are close to £6k per term each without extras. I expect to pay £19k per term this year, rising to £20k next year.
We are not rich, no inheritance, no grandparents paying, no city bonuses. One full time professional salary (just into 6 figures), one part time salary around £20k.
We can do it because we have always planned and worked towards this from the early days of our marriage. In our early careers we always lived well within means and saved all spare cash to pay off the mortgage. Once established in house number two we continued to save as much as possible. When we could have afforded it, instead of buying a bigger house we stayed put and bought a BTL which could be sold if we need the cash. The mortgages are both low tracker rates secured just as our first child went into secondary, and just before interest rates plummeted- this has made a massive difference to us.
We kept extra curricular expenses to a minimum- eg taught all dc's to swim ourselves, group music lessons rather than individual. Family meals out are a treat rather than a regular thing etc.
We are not mean - just sensible and careful with our money, and I keep a detailed record of finances and outgoings to keep me sane. We can cover pretty much two sets out of income with the balance from savings, and the savings shouldn't run out before we are done.
So this is how we have done it - very long term financial planning some good luck.