Which would be the better word to use for short, informative "films" made to attract students to a university – film or infomercial? Or something else entirely? Perhaps I could use promo? But I need it for a semi-formal text, so perhaps promo is too informal?
"Promo" is short for "promotional video". These can be informative and they are intended to attract students to the university.
An infomercial is something that is shown on TV. If the university has purchased time on (for example) the Home Shopping channel, that would be an infomercial. But that would be unusual. Most infomercials are selling kitchen or gym equipment.
The difference between an advert and promotional video is mostly how it is broadcast. For example, if will see the video on your website, then it a promo, but I am going to see it in the gaps between videos that I want to see on youtube, then it is an advert.
Promotional video would seem to be the best, and fairly formal expression.