Wow. This question triggered some long forgotten memories! I used to work in a student computer lab where we wanted to do just this. We had a mix of LCIII's and (new) PowerMac 6100's. We did in fact get this to work using "The Diskless Mac" from Sonic Systems - which was essentially a boot ROM that replaced the onboard ROM on a PDS or NuBUS (was the 6100 NuBus - I cant remember) ethernet card.
Google "The Diskless Mac" with "Sonic Systems" and you'll see how it works. We did this with System 7, but I have to tell you that we abandoned the idea not too far into it - the problem was when you had 40 machines all reboot at the same time - veeeeeeeery slow. A few machines at a time was fine however, so not likely to be a problem if you can find any of this kit
In the end we went with a product called RevRDist. This allowed you to keep your local HD synchronised with a image on the server - much faster user experience, but less secure - it ended up being the right trade of tho.
Cheers,