The Malory Towers series is definitely 7 year-old level. The set is on offer with The Book People for £4.99 atm here, although I'm not sure of their overseas delivery policy. I can't remember the St. Clare's one I'm afraid.
The other school series I can remember are: The Worst Witch (age 7-9 I'd say from memory) The Chalet School (a bit older) Trebizon (age 9-10)
I know! I was in Denmark a couple of weeks ago for a series of meetings and my flight was delayed on the way home so I bought a book to pass the time - that it was over £15 for a bog-standard paperback that would have been a fiver max here!
I would second Mallory Towers - I remember enjoying it more than St Clares, though to be honest they were very, very similar! AKMD I had totally forgotten about the Chalet School - I used to love those books, and thought all the girls were really chic!
My mum had 1950s school stories that I borrowed when I was little (the Abbey School?) and I just loved those. It's so nice to share something you enjoyed as a child yourself.
DD read St Clares first (only because I had read them first as a child as someone passed them on) followed by Malory Towers. I don't think it really matters. Milly Molly Mandy and My Naughty little sister books are good for that age, a bit old fashioned in their writing but no more so than the boarding school books, and DD loved them too.