I have a yr8 DD & yr7 DS
DD has: 2 Blazers (1 is secondhand & kept in the wardrobe as a back up)
3 skirts, 1 trousers, 2 ties (1 kept with spare blazer), 6 shirts, 2 jumpers (she does wear them), 5 thermal tights, 3 socks, 5 thermal t-shirts.
PE kit - 1 skort, 1 shorts (rarely worn), 1 leggings, 1 t-shirt, 1 hoodie, 1 socks, 1 trainers.
DS - 2 Blazers (again one is secondhand) 4 trousers, 6 shirts, 2 ties, 6 socks, 5 thermal t-shirts & 2 jumpers.
PE kit - 1 shorts, 1 t-shirt, 1 rugby top, 1 socks, trainers, football boots, shin pads & mouth guard.
My top tip would be make sure the rucksack they take is big enough for everything, PE kit, lunch box, cooking ingredients if necessary. I bought rucksacks of 30 + litres for both mine & it's been so much easier for them. They started with smaller & lost their pe kits (& found them again but it caused them stress).
Lunch boxes are just basic plastic tubs by Sistema or similar.