There is a problem with managed clients and external displays: The user (a high school student, for example) can change the display to an unsupported resolution/frequency, rendering the display useless. When this happens, it's broken for that user on that machine, because of the
By Host preferences folder. (What's with that folder anyways? I mean, why?) I know there is that warning and confirmation that will auto-revert, but sometimes it doesn't warn, when it 'thinks' it will work. Anyways, the kids manage to make the display black.
You can always lock out the Displays prefpane with managed preferences, but they can still switch it with the menu extra. So the simple solution is to take away read access for "others" on the menu extra:
sudo chmod 750 /System/Library/CoreServices/Menu Extras/Displays.menu/
[
robg adds: This older hint explained one way to recover from invalid resolutions. I'm not sure if it's still applicable or not...]