how to limit access of wireshark to 1 of several nics

Unfortunately that's not possible without a change to the code of Wireshark. See my answer in the question below:

http://ask.wireshark.org/questions/14067/limit-interface

Furthermore: Even if you could limit Wireshark to listen to one specific interface, your students could just swap the network cables and would then still be able to capture traffic on the campus network.

So, what can you do:

• build a virtual lab in vmware (or KVM, Xen, etc.) and let the students connect to the capturing machine via RDP, VNC or any other remote desktop software. You can easily separate the virtual lab network from the campus network in a virtual environment.
• don't care too much about the students sniffing the campus network. They can do that anyway, if they bring their own laptop to the campus ;-)
• don't care too much about the students sniffing the campus network, as they will only see their own traffic and broadcast traffic if they are connected to a switch, unless the students use some arp spoofing/flooding tools (maybe you even teach them how to use those kind of tools in your course). So, you should just forbid using those tools on the campus network, as they can severely disrupt the network operation.

Regards
Kurt