Children’s book (1970s?) about a young girl witch getting a witchy education at a school for little witches

(NOT ‘The Littlest Witch’) Very simple line illustrations, mostly black with some blue and or green, lots of white space. The ‘school’ is somewhere very high up (mountain? cliff? floating island?) only accessible by broom. I think there is maybe only one Elder/Teacher witch, and maybe a handful of other students? Maybe just the little witch? She has a black cat as a familiar, and has to learn to fly, maybe from some kind of intimidating platform. I remember it having kind of a lonely feeling (the Elder witch is not her parent, maybe at first she is at this ‘school’ reluctantly or unexpectedly) It’s not spooky, nor overly cute.
Left a hazy but rsomehow strong impression.
Any ideas…?