The exam fees hit us unexpectedly, so it's good that you have had warning - especially when DS chose to do some entirely unnecessary resits without letting us know...
Grandparents took over music lesson fees as birthday/xmas presents. As it was preferable to things they had been choosing as gifts, no pouting or lip-trembles were involved. 2nd hand uniform shops, get there early. Also scour eBay. Go to the supplier, have a look to see what the school really need you to buy, then go and buy cheaper trousers/shirts elsewhere. It's a bit harder with girls stuff but not impossible. They usually have some weird impossible to track down tartan skirt. However their incredibly expensive socks and tights are often to be found elsewhere. I did buy new blazers for them though, it drew attention away from the fading elsewhere!
Be aware that if they choose to join the aqua-diving after-school club, they may be expected to join the summer trip to the Red sea... in which case, bang goes the holiday for the rest of the family. Research is key, but they WON'T be the 'only one not going', whatever they say.
Din into them that they cannot waste paper, lose pens or folders, because you will be charged for them. some schools charge for everything, some for only replacements. If they lose a text book, you will also pay for that, one year I bought a cheap eBay replacement for every book my scatty DS was given. It was the only year when he didn't lose a single one.
However, you won't be paying out for packed lunches, most clubs are free unless it's riding or something off-site, and DS joined CCF which meant that he got a week's activity holiday every year for around £85. I would say that Geography is probably the most expensive course to take, as it involves many field trips. And care in the community club can result in £3000 trips to Malawi to dig wells for schools in yr 12, so get them a job. Now, preferably.
But you know what, you do find it, somehow. The price we paid for it was smaller houses than we would have liked, doing work we didn't like sometimes, but it's all been worth it. The last fees were paid last year and I wouldn't have changed a thing. Mine went through all sorts of schools as we moved around a lot, both state and private, but lovely though some state schools were, the private ones won hands down.