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.
In the parking lot when Alexia (Hailey) is driving erratic the Hunter High sign has "Morp April 12" on it which was happening that weekend at Hunter High.
Alexia and Hayley both get in trouble for misconduct in the cafeteria. Hayley (as Alexia) gets in trouble for making out with Kyle during Alexia's striptease (as Hayley), as well as bragging loudly that he gave her (Hayley, as Alexia) a hickey, but Kyle is never even threatened with any consequences as the girls are, even though he is just as guilty of misconduct.
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.