I'll assume your school has Windoze installed on the lab computers:
1) Leave your computers on the network to keep your virus updating software up to date. Why? Because students are going to place in USB drives/devices/etc that may be ALREADY infected. Trust me on this.
2) Make sure the students' accounts are NOT administative accounts
3) Install a free virtual PC system, like's SUNs Virtual Box
4) Install Windows98 in the Virtual Box System.
5) Look for a virus that existed BEFORE 2002. Some current viri can actually penetrate Virtual Machines (VMs), and if you think you are a smart pants and install a VM inside a VM, these viri can penetrate right to the main, "real" OS. Viri before 2002 are very likely unaware of VMs. You can get the Virus list from Nortan (SYmatecs) websites.
6) Now the tricky part. Finding the original virus source from a "reputable" website. :)
7) Copy source into VM, compile and run.
8) Have fun!