When I was studying design and had to carry a lot of supplies I'd go with double panniers BUT my faculty was very small so I didn't need to take them off the bike. Other days, I'd just strap my backpack to the rear rack with bungie cords.

Given my experience and considering her campus may not be as secure as mine was, I'd go with something that had a quick release mechanism and was easy to carry off the bike (bonus points if the material is easy to clean).

Ortlieb panniers are highly regarded as they're waterproof and with a quick release system. They even have an add-on that lets you use one as a backpack.

Newlooxs and Basil both offer many options of panniers that can be used as bags off the bike.

But, yeah, you'll need a rear rack, which is easy to add to most bikes.