Because . . . the Beast and the monks used their own particular spells, which had dissimilar side effects.
In other words, a different wizard did it?

SPOILER?
In the books, the villain basically paired the adults up and told them all to get pregnant, in order to have complete control over the next generation.  It will be very weird if they go this route, but yeah, otherwise I don't know why every witch is in the same grade at school.
Also, Sally is in the books, and…

Those were great - Bruce Coville wrote a couple of them, and there was one Lovecraftian one called The Ashton Horror.  They were really dark for kids books (or so I remember).