I'm a middle school teacher and I have exposed my students to the wonders of computers (UNIX, PCs, and Macs.) My school has a new Mac lab with 30, 1 year old iMacs (not my choice, 40% hardware failure rate). We also have a butt load of the older ppc g3 & g4 Macs. I would like to turn a dozen of these older Macs into Linux servers so I can teach the students the beauty of open source computing, advanced techniques in networking, web hosting, etc. I can (and will) also let then hack & slash the servers because that's how we learn. Anyway, I'm looking for recommendations for which DISTRO to use. Remember, these old Macs typically sport a CDROM drive, 512mb ram & 40g of local disk storage.

Any recommendations??

First recommendation is to check out distrowatch.com. They have a very good search engine for that. But, one that comes to mind is "Yellow Dog Linux". I used it a few years ago. Checking distrowatch, it still looks active. I had no problems installing it and using it at the time.