To to my favorite WEB Search Engine, AltaVista, and enter in +"occupancy sensor" (use the '+' and "), and press Search. You'll get TONS of links for Occupancy Sensors!
One in particular is from Sensor Switch, INC.. They carry quite a few of varieties. From their Home Page, you'll find 3 links for Restroom (Bathroom) Applications. The first one, Model WSD-PDT, is interesting. BUT, I don't see a manual ON or OFF capability which is something you want for security purposes. (IF the light doesn't come on, you want to be able to see if some wiseguy/wisegirl turned it OFF.) Also, I don't know if it's X-10 compatible. (Talk to anyone that's computer-literate to explain this capability. I know what it is but I'll spare the explanation on the Forum.) This particular model says it's ALMOST foolproof. What does THAT mean?! So, when you're out shopping around, make sure you line up all your P's and Q's on these types of questions. One good thing about this particular model is it works with flourescent AND incandescent lights. Again, a 'characteristic' you want to note. The length of the 'on-time' is another feature you want to note. Once the light comes on, how LOOOONG will it stay on? This options should be variable and able to be set by the owner. For you, this is important if you want to save energy. Flourescent lights are more efficient when on for many hours BUT they suck up a LOT of energy EVERY time they come on. As for incandescent lights, they are very INEFFICIENT when they're on for a long time BUT they use nominal energy to turn on. So, if a room has flourescent lighting and you set the 'on time' for 5 minutes, it will be going on a LOT during the day and using a LOT of energy at the same time. If you set it for 15 minutes, you'll certainly improve your energy effeciency. BUT, this is something you'd have to research. (Maybe you need to sit outside the bathroom and see how often someone goes in.) DO understand that if the bathrooms have showers, some people take showers that are over 15 minutes, and if they're taking one at 3am and the lights go off, well, you're gonna catch a lot of hell!
I think you idea is an interesting one. I've seen these types of switches used in bathrooms that don't have showers, just 1/2 baths. Unless you can find a timer that's capable of being set to 1/2 hour of on-time, you might have a tough time convincing the administration that these types of switches are good for dormatory shower-rooms/bathroom. Plus, everyone will pretty much have to be educated now that they don't have to go reaching for a switch every time they walk into a restroom. This could be more trouble than it's worth. Perhaps focus on rooms like 1/2 bathrooms is best.