Just make this a test question for your test takers doing MCSE, CCNA, or even Net+
Actually, this might be tough to resolve. Upon the next failure, you should try some tests to determine more precisely what is failing.
1) Ping from the Admin computer to the failed test computer. Ping reply received?
If yes, Try to access a share on the test computer from the Admin computer (note: ensure you can access the share before failure)
If no, check the connectivity lights on the hub/shwitch/and related NIC. DO the show connectivity and activity like before the failure?