Right now, in order to do this, you have to enable the ability for anyone to be able to remove themselves from the directory, remove the user, and then disable that ability.   In the meantime, if anyone sneaks in under the wire, (this includes students!) then can disable their info in the directory, which will then stay disabled until someone notices they're missing.  On a practical level, this means that no one at the school has access to their contact info during that time frame.