We had the same problems. The blazer is huge. My dd2 is apparently average height - but not tall - for her age, but skinny. The blazer hangs off her like a sack. The PE trousers were about six inches too long and of the kind of material that could not be taken up (sort of a shell suit, with logo, obviously). The trousers - thank goodness - don't have a logo so we got them elsewhere.
But the skirt looks terrible. It is just too big. She must be thinner than i realise or something. One of her friends took a photo of herself and she had the same problem, all the uniform is too big. I don't know that it's a size 10, but I doubt it was smaller than an 8.
The shop assistant just said she'd "grow into it". In about five years, I thought
Naturally it wasn't exactly cheap, either