Other than Nick, who could be the snitch? This isn't the first illegal party the DIKs have hosted, so it probably isn't the nerds. The jocks were unaware of the party until the next day. Preps don't give a shit. The dormies hold a grudge of course, but even Kylie still showed up to the party.
The DIK frat has been around for two or three years .. at this point, anyone at B&R who has heard that there is a party going on at the frat, can pretty much take it for granted that alcohol and drugs will also be present. Whoever told Professor Hoff about them doens't necessarily know what is happening there - they may just have inferred from what's publicly known about the frat to begin with.
You can suspect anyone who has even heard that the party is happening. They don't need to be on the guest list, they only need to have heard from someone else about it taking place. Who, logically, has a motive now that hasn't had one before, that is a lot more difficult to work out. It could be the nerds if they are stepping up to a new phase of whatever the hell Magnar's scheme is...