If you want to encounter students in Helsinki in their natural habitat (outside of parties, I mean) and/or are travelling on a budget, you should stop by Unicafe Ylioppilasaukio. It’s basically like a university canteen but in the city centre and open to everyone.
Finnish students pay ridiculously little starting from € 2.40 (in other schools sometimes even only € 1.35) for a full meal, including a main dish, salad, drinks and bread. As an “outsider” it is a bit more, but still at a lower price than in any other restaurant to be found around. The food is not the best you’ve ever had in your life, but for the price, nothing beats it.
As a student here myself I am often stunned by the quality of the food, fresh ingredients, fish, whole grain pasta and rice and rarely anything fried and greasy. There are always various options, alternatives for vegetarians and vegans are also available. It’s not the most awesome food ever, but if you’re looking for an inexpensive meal, this would be your place to go.