You have to look at the board. If they are in a case , you might be able to get around this by taking a look at /var/log/messages on the server as they attempt to boot up - I believe the requesting MAC address shows in the DHCP request message that gets logged.

You could not use MAC addresses, and simply allow the boards to get assigned an IP, but......

Maybe my system was mis-configured before, but I found that in that case they didn't end up knowing their host-name. If you want to set up host-specific directories for them to log into on the server, this is a problem....

(You can maybe do it via IP address instead of host-name? Hmmm. Interesting idea... )