As someone who's been through this before (the now-defunct studentuk.com and gradunet.co.uk, 5 years ago) plus running a whole bunch of student print / radio media from way back, a few tips....

1. CPM averages you will be looking at are going to be about £1. Start with an agency to reduce risk unless you can find a salesperson with astonishing media contacts who'll work mostly on commission.

2. Forget CPA. When this agegroup buys, it's not going to be after clicking on your ads.

3. Mediabuyers are not actually interested in students (apart from banks for a few short weeks in September and graduate recruiters for the whole lifetime of a student). Outside of these 2 sectors you are fighting the more general yoof sites (MTV etc etc) for every ad dollar and it is a hard fight. Banks are stingy but graduate recruiters are astonishingly lucrative. But people like milkround have got that market largely sown up.

4. All the student websites from 5 years ago are dead (even virginstudent which was horribly well funded), and most of the student marketing agencies are as well. It's a hard market - I'd focus less on the student and more on "young adults". Mediabuyers do, because they know they can hit more students on facebook than any other type of student media (even nusonline)

5. Talk to some people who really know this market - try luke @ http://www.reachstudents.co.uk/ and the folk at http://www.campusgroup.co.uk 

HTH, (not trying to be negative but you really need a strong USP in that market!)

i