When Zelda goes to talk to the teacher Hilda is stirring the cauldron but in the next scene she is a few feet away from it.
Sabrina stops time in order to study for test without any consequences. She could have done the exact same thing in order to take her test.
In the pilot, a big deal was made out of how witches are not allowed to time-travel to change the course of events. But now, before the maths test, Sabrina freezes everybody else in time to look something up, which basically amounts to the same thing.