some of the bandwidth meters have provision to stop the net connection after a limited traffic ,the limit can be adjusted to 20 Gb
few proffessional versions have provision to run any command on reaching the limit .
it may be a simple solution for your problem.
but 20Gb is not much for 16 peoples' regular usage of internet for normal usage in a university.
sinple email checking ,google search and few articles downloads will easily reach 1 to 1.5 GB per month for onq single person.
almost for the same money you must be able to get an unlimited download connection.
i feel the students need that and more even without movies or any undesirable downloads from net