Another idea to consider would be to add another wifi router, one configured
with wifi security (WPA) and setup solely for use by the guests. Then add
the traffic monitoring, MRTG works well. It'd even be possible to track the
live packet usage from the separate router and CUT IT OFF should the traffic
exceed certain amounts.

It'd end up being a bit of a configuration adventure, but you could do this
with an old PC running linux and two ethernet cards. The PC would act as a
'traffic cop'. Plug the student wifi router into one of it's ethernet
ports, and plug the other into the ISP switch. Configure the student's wifi
router as an access point, not as a gateway or a router. Then configure the
PC as a router. Then just configure the scripts on it to cut off the
ethernet port of the student wifi rig. You could get all sorts of fancy
with configuring it. You could even set it up to re-direct ALL web traffic
from the student port to a web page showing traffic stats if/when they run
over the limits.

So no, there's not one box that'll do it, at least not at a reasonable
price. But a cheapie PC with linux and some configuring would do it quite
well.

-Bill Kearney