When Alexia answers the Chemistry question in class, she writes "ABC" on the whiteboard. In the next shot as she continues writing, the appearance of "ABC" is completely different.
When Alexia (as Haley) is grabbing the boombox before school, it has two tape decks, but when she uses the boombox at school, it only has one tape deck.
The morning after the wish is made, Hayley and Alexia notice the switch when they're in the bathroom. Naturally, they both scream but neither of their parents rush upstairs to find out why. One could argue it was part of their "hands-off method" but that shouldn't eliminate being concerned about screaming.