For a building of that size I would have thought the meter would have a pulse output.
This can be monitored constantly and show when most usage occurs.
eg ;
Crucible Technologies - Home
Failing that,
Check the meter every hour through a 24hr period.
Plus,
Does each riser have it's own isolation valve?
If so, someone watch the meter, someone turn off each riser 1 at a time.
The flow through the meter will disappear when the guilty riser is found.
It won't affect the users if you turn the riser off and then straight back on.
Maybe do it early, before 10 am before the students get up !!
Then track down users on the guilty riser.
If it's spread over all risers, you must have normal consumption and the other building has extremely low consumption.
PS beaten to some of it by Chalked