I was surprised how hard it was to find such a thing. Most of the search results I got pulled up complicated network monitors intended for network administrators.
You might try vnstat.
Then initialize the database:
sudo apt-get install vnstat
Change eth0 in that command to the name of your network interface.
sudo vnstat -u -i eth0
I haven't used this program much so I don't know all the details, but these commands seem useful:
vnstat --> Shows today's usage.
vnstat -d -->Breaks down stats by day.
vnstat -w -->Breaks down stats by week. Probably the one you want for your situation.
vnstat -m -->Shows stats by month.
sudo vnstat -u --> Force an updates of the database.
The vnstat homepage has more info here: http://humdi.net/vnstat/
Hope that helps. It doesn't seem right for the hostel to impose that policy and not give you an easy way to check your usage.