Other mistake: During the 3-nightmare sequences, there are 25 students, with all desks full. However, 1 desk should have been left open as Kevin appears in the dreams, meaning there should only be 24 students.
Factual error: During the scene when the Doors' "Riders on the Storm" is playing. This scene takes place in late 1968, early 1969. "Riders on the Storm" would not be written for at least another year, and the Doors album with that song was released in early 1971.