Slightly pointless question but does anyone see the point of all these larger size uniforms for bigger than average kids. You do need larger sizes for some, but BHS for example has their larger sizes for age 4 the same as about age 8 regular (so why not just buy an age 8). My DD is 13 and usually wears age 15 or 16, the generous fit trousers or skirt for that age fall right off her. BHS even have and age 12 gingham dress supposedly adult size 16, like a p7 pupil would be that big (or a 16 year old would wear these things). M&S have a Plus Fit, which I could say the same things about.
tbh if a teen is so big they don't fit childrens' clothes, why not buy adult shirts, trousers, skirts etc. in the appropriate colour. School specific items like badged jumpers can be specially made if necessary.