If it is really a peer-to-peer communication which doesn't use your wireless AP, I'm not sure it is legal to monitor that communication even if you don't decrypt its contents.
Technically - on Mac and some other machines, you can use monitor mode of Wireshark to capture the traffic in the air, bearing in mind that it is a radio communication so you may not be able to capture every single packet - there are two bands, many channels in each of them, many modulation schemes etc., and for peer-to-peer communication, the machines may use different settings than those they use for network connection by means of the wireless AP. As the peer-to-peer communication is likely to be encrypted, you are unlikely to see IP addresses directly, but you might be able to see MAC addresses which are not encrypted, and it is likely that the same MAC addresses of the WLAN adaptors which are used for peer-to-peer communication are used also to obtain IP addresses from the wireless network's DHCP so this relationship should be available there. I assume the students' machines authorize using individual credentials, each stuck to a singe IP address, otherwise I cannot see how knowledge of the IP address should help you.
