T.S.
No need to get it all now (or ANY of it now actually). Pick up enough season appropriate pieces in the correct size at back to school time for you to feel like they have enough to get started. When it starts cooling off, get more pieces.
That way they'll always have clothes that fit, and you won't end up stockpiled with things they end up not wearing. (for example, if they're running the heat inside and wearing coats outside, you might not WANT them in long sleeves at all, but if it's cold in the classrooms, they may need a couple of cardigans to wear in class. You won't know until they're there).
It won't matter that their wearing the same thing every week... the whole point of limited dress-codes/uniforms is to take the attention OFF of clothing.