From your previous experience with the IT department I would simply forget about them. Gather all your evidence, logs etc. and go and stand outside Peter MacPherson's office until he talks to you. Be prepared to explain in depth but in laymen's terms.....

Point out to him that the network is heavily infected and the potential for a dangerous information leak is quite high. That information leak could be of the kind that would leave the university liable and that is probably something he doesn't want. Furthermore, the reputation of this particular, (well known), school would not be improved at all should the fact that the network is so infected that it takes mere seconds for a student to become infected after connection to the network become public. Add the fact that the IT department are in denial and that information would guarantee that parents think twice about sending their hard earned money there when there is a perfectly good school just up the road.

Don't make it sound like a threat to disseminate the information but just that fact that you are a concerned student interested in the well being of the school and it's students and you think that this is something that the IT department should be a little more proactive about.