Have you considered identifying groups they are involved in and asking them through that group? There could be a Facebook group for their University or possibly even a Google + Community? Facebook ads can be targeted by many factors, and a university might be one of them. (You'd have to consult an expert on Facebook advertising to find out. I am not one, but I may know others who can tell me.)

Also search LinkedIn for groups related to specific universities. You can often join such groups and interact yourself or you can identify leaders and contact them to ask whether they would be willing to tell the group about your test.

You could also consider advertising in the school paper. Most Universities would have one which you could find by typing in "University paper" in any search engine or by using specific university names in your search, i.e.: "Yale University paper", "USC university paper", etc.

Other ways you could find students are local free classifieds such as Thrifty Nickel, Pennysaver, etc. or by posting on Craigslist.

You could also create something so entertaining it would go viral to reach them (often done in conjunction with Facebook), but that can be a major challenge not easily done without a substantial budget.

Tom Williams has the best idea though - in person outreach in the manner he described will work best. I went ahead and shared these other ideas because I suspect you want to do many campuses and may not have the budget to travel to them all.