@Markgraf That, or she has to lick a shadow that's connected to her target's, since there was a detail of the girls' shadows stretching into to the darkened space the teacher was hiding in.
@gouder here is my guess:
The spell she memorized only worked due to the interference of an oddity. So far, the teacher's oddity has only been shown to work against humans, not other oddities. As such, I think by licking the floor she either "activated" the oddity making her a child, or "deactivated" the oddity that allows her to go back to being an adult