After some thought, I think you're right. Something about 'wizard' is just more socially acceptable. Harrietta the Witch just holds all sorts of negative connotations. Throughout the history of fiction there have been good witches, but no one remembers them. Everyone always concentrates on the very bad ones. Right? Your eye is drawn to the Malficient, the grotesque, the wicked witch of the west. Wizards are always Baalam or Gandalf or some other dottering old wise harmless thing. That's what I love about this series of books. It shakes up all the preconceived notions anyone might have about magic. Or school in England.