1: Michel (The Principal) and Nicole (his mistress) were really planning to kill Christina (Michel's wife) the whole time.
2: The gotcha - The Commissioner enters immediately the room immediately after Michel and Nicole exploit Christina's weak heart to kill her, and presumably arrests them for murder.
3: Christina is apparently still alive after the murder, because Moinet claims to have seen her.
I think in order for the film to make any sense, 3 must be true, because otherwise The Commissioner knew of Michel and Nicole's plot and let them execute it anyway. How she's not dead, I have no idea, since Michel checked her pulse, and there's certainly no indication that Christina wasn't actually scared to death as she was running from room to room.
I suppose The Commissioner could have decided everyone, including Christina, deserved what they got, since the are all essentially murderers (Devils!). That The Commissioner would allow a battered woman to be murdered speaks of him as a sick bastard.
Regardless, the film does seem to punish Christina, either with death, or with taking the school away from her.
No sure if I said everything I wanted to say there...