m&s clothing

[BeautyQueen22]

I'm looking at ordering some uniform for my 10 year old son from m&s and I'm just wondering if anyone can tell me what the sizing comes up like please?

I took Ds 2 aged just 14 to M&S last weekend to buy new school trousers. He is small for his age but quickly catching up. He tried on the trousers aged 11-12 and they were fine. I bought him the same size shirts ( he didn't try those on) and went back and exchanged them today for 12-13. We are a quite short legged family