It took the recent death of Diana Wynne Jones for me to revisit her work after many years and the first of the Chrestomanci chronicles (Charmed Life - 1977) is as good a place as any to start.

Rereading with post-Potter eyes is a revelation. In a 2003 Guardian interview the similarities were evident even to children:

But children reading the reissued Chrestomanci books soon commented on the likeness to Harry Potter: school background to the magic, referring to a character as one who must not be named. "I think," says Jones, "that she [Rowling] read my books as a young person and remembered lots of stuff; there are so many striking similarities."

DWJ was most generous in her response.

Charmed Life heralds a far better written series than HP and should be put into the hands of young fantasy readers quick smart. It's an engrossing read and they will quickly devour others in the Chrestomanci series.