[ninty]

Maybe he cocked it and then took the mag off. That would result in a round in the chamber, but your right, if he took the mag off first and cocked it, the round should be ejected.

the only other thing I can think of is if the round isn't ejected until the slide is put forward again. If he cocked it back and asked the student to look in it and then it went off, that is possible. Thats why you cock twice after removing the magazine. The first ejects the round. The second you can look in the chamber to make sure there isn't anything there.