I know what you mean with student-y cafes, but a lot of the students I know here hang out at Starbucks.

These two are as close an approximation to a student cafe from home that I can think of:
Zaehringer, near Zentralbibliothek (operates as a cooperative)
Schoffels

There are a few Indian/Thai sit in/Takeaways that offer good sized portions for 10-15 francs in the Wiedikon/Langstrasse area.

Paddy Reilly's offers 2 for 1 on all meals over 18 francs on a Monday from 18.00-2200 and the food there is very good.

McGees Irish Pub in Wiedikon have a fantastic and authentic all you can eat Indian Buffet on Monday nights for 25 francs. Have a BIG bowl of cereal for late breakfast and head there for all you can eat Indian from 18.30 til around 21.30/22.00.

Manor has a large buffet system all day where you choose your choice of meat, and then fill the plate with help youself salad/warm buffet. 14 francs for chicken breast and plate of veg and salad for example and it's consistently good. One serving, you can't go back for more.

Co-op and Migros offer good value for money, but aren't as good as Manor in my eyes.

Studers - http://www.studers-restaurant.ch/ - very close to Esher-Wyss Platz seems to attaract a lot of students / young folks and have very reasonable prices