the no magic rule

witch u was only for witchs at one time but then thay started to let every one in and thats way thay have the no magic rule.

The fact that Marnie caused the magic rule didn't happen in the fourth movie, it happened when she opened the door between Halloweentown and the mortal world permanently, letting Halloweentown citizens go to the mortal world and vice versa as they please. Aneesa, the genie, explains to Marnie that Witch U used to only be for witches. But since she opened up the door, a lot of the students were going to live in the mortal world, thus dropping school populous quite a bit. Needing students, Witch U decided to open up the school for non-witches (ie genies, mummies and the like) who couldn't use magic. They thought it would be unfair for witches to use magic to their advantage when other "races" couldn't, so they made the no magic rule.