Yes, they're available in many locations, including Amazon, Newegg, and even eBay (probably the least expensive). And you don’t need to spend a lot (despite the occasional nutty prices you’ll see), $7-15 is plenty. They’re all pretty much the same, just make sure it’s USB 2.0 and 10/100Mbps minimum (almost all are anyway).
http://www.amazon.com/s/ref=a9_sc_1?rh=i%3Aaps%2Ck%3Ausb+ethernet+adapter&keywords=usb+ethernet+adapter&ie=UTF8&qid=1343685034
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=-1&IsNodeId=1&Description=usb%20ethernet%20adapter&bop=And&Order=PRICE&PageSize=20
P.S. I should add, if you prefer, you could get a wireless AP instead (even a wireless router) and patch that to the wireless ethernet port provided by the school. That way he'd be using wireless rather than tethered to some stupid ethernet cable. The only reason the school says ethernet is that they don't intend to provide wireless (saves them a little money). But nothing says you can't bring your own (assuming, of course, the school doesn't have a policy against it).
[ethernet @ wall]<-- wire -->[wireless ap]<-- wireless -->[laptop]