There's been a thread recently on the Green Schools List about the impacts experienced by colleges and universities which have removed the traditional cafeteria trays from their dining facilities. Results range widely.
UC Santa Cruz (enrollment 14,400) reports that they're saving 20,000+ gallons of water (presumably, heated water) per month. The savings result not just from not having to wash trays, but also from washing fewer dishes -- fewer instances of students grabbing an extra dessert which they may not eat (and certainly don't need).
St. Lawrence U says they're saving 23 tons of food a year. With an enrollment of 2200, that's about 21 pounds of food per student. (Note that both UCSC and SLU house almost all their undergrads on campus.)
The National Wildlife Federation's Campus Ecology program published a study of a half-dozen schools who reported both positive and negative responses from students and staff. (Negative responses from students in dining facilities -- you can do that one in your head.)
On the cautionary side, Ohio U reported food savings which, statistically speaking, disappeared into the noise level. Results are described here.
So, in a nutshell, we don't yet have good data on what sort of input reductions (food, water, energy) a campus can expect from elimination of cafeteria trays. Administrators like me tend to say "do it anyways, what's it going to hurt?" But there's clearly a research opportunity here, and an evolving set of data upon which to base that research. What characteristics contribute to successful (or unsuccessful) input reductions from tray elimination? Is it about campus culture? The food being served? Other steps (signage, etc.) taken at the same time? Are the apparent differences really an artifact of measurement error? What?
If I were a Masters' student in a sustainability program or a Senior honors student, I think I'd be looking at a great thesis topic here. And I wouldn't even have to go off campus(es) to get my experimental data.
Meanwhile -- for students at schools which still provide trays, I suggest you grab one now. There's snow on the ground (at least, in the northern USA), and it's tray-sliding season! (Hint from an old hand at this -- coat the tray bottom with car wax -- you'll go farther and faster.) (Oh well, so much for getting the dining hall managers on my side! <lol>)