First off. Your profile says you are in Long Island, NY. If you have not already done so, RIGHT NOW go to
http://www.photoplusexpo.com and register for the expo next weekend (Oct. 22-24) there is a vip code for a free expo pass, but I believe it's a violation of dpreivew.com's forum rules to post it here. If you need it send me an email and I'll give it to you.
The Expo is a great opportunity to meet with representatives for each camera company. Nearly all companies have sales reps that focus on educational sales. If these cameras are going to be used by a photo department for students, they may give ENORMOUS discounts, or in the case of Nikon they may offer a reasonable leasing option. Go to each booth at the expo, explain your situation that you need 20 cameras for an education program and they'll tell you "oh you should talk to Bob..." and either get the person you need to talk to or at the very least give you their card. If you can go on Thursday or Friday, I would recommend it because on Saturday the booths are swamped with people who are going to buy a point-and-shoot but want to play with the D3s or what have you and it's harder to get someone's attention. I work for a very large university, know a lot of people in the industry, and last year my department spent well over $100,000 on equipment and I still have to fight to get the time of day on Saturday.
As far as which camera? I'd hit up Canon, Nikon, Olympus, Pentax, Sony, and Sigma and see if any are doing educational programs. If you have time talk to Leica, MAC Group, Hasselblad, and the others as in the past they have given scholarships and maybe one camera or something, but I wouldn't hold your breath in this economy.
Even if you can't make it to the Expo I'd make a lot of calls and place a lot of emails.
Canon, Nikon, Olympus, and to a lesser extent Sony, Pentax, and Sigma actively try to get high school and college photography programs to use their cameras because they know if the students use their cameras in school they'll get used to the camera and that's what they buy when they are out in the real world.
Any one of those camera system would be good and I've seen people in this thread talking about how you should get IS/VR or the best ISO and while it's cool to have, it's one more thing to deal with so I think it's a wash. You want the students to be able to focus on Exposure, Focus, White Balance, and Composition.
I wouldn't worry about which camera to get as much as what camera you can get the best deal on. Any dSLR today will work from the SD15 to the D3s. Talk to the companies and see what's the best offer. Yes, if Olympus gives you the best offer you'll have to spend a little time learning their menu system so you can teach the students, but it's not a big deal and the students shouldn't be spending too much time in the menu system too much anyway, they should be taking photos.
But I would definitely go to the Expo, even if every company is crying broke from the economy and says they can't give you anything, at least you can play with each camera system and see which you feel it the most intuitive and least intrusive to the act of photographing. To me the camera should be as transparent as possible.
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~Kurt