get the biggest dry bag you can for a rucksac liner - sports direct do very big, quite cheap ones. I'd also recommend a couple of smaller ones for sleeping bags and clothes.
Pot noodle if definitely not a sufficient meal for someone hiking miles and carrying all their kit, and the wayfairer type meals are the easiest thing to ensure no spills/ leaks, and no food poisoning if not cooked properly. Go Outdoor do some of their own versions (DofE participants also get a further discount on top of the discount card price - ask in store about it). I'd also recommend porridge if they have time to cook breakfast, and flapjack, tiffin etc as snacks.
The costs likely include a company with trainers and examiners to run the sessions, which will be the bulk of the cost, campsite fees, a contribution towards the school issued kit (assuming tents, rucksacs, trangias etc are all provided by school), which would be something most scout units would be running on volunteers and already have the equipment to lend out which had been paid for and used several times a year, with everyone paying a minimal cost for upkeep.
If you're keen to keep costs down for everyone, how about volunteering to get trained up to take a group out every year? If you can rope a few parents from other year groups in, you could definitely help the school to keep those costs down! Similarly, if you school doesn't have a mini bus or teachers to drive it, could you offer a lift to others to help keep everyone's petrol costs down?
If you did it through scouts, don't forgot you'd also have scouting subs to pay, as well as any additional fees aside from DofE (I know someone who did it through scouts for a similar price, but when it was broken down, parents were also paying for the meals, and a separate fee (£20 ish) for each expedition which covered campsite fees etc, and the initial fee just covered the scheme cost).