I've never actually read the Worst Witch books, but I remember the TV series and I loved the movie with Fairuza Balk and Tim Curry. And yeah, that was one of the first things I thought of when I heard about the Harry Potter series.

I also enjoyed Diane Duane's So You Want To Be A Wizard series. Erase and correct; I liked the first three books. After High Wizardry, the series starts being less about magic and more about physics, which kind of sucks the life out of the books, IMO.