I am afraid I cannot help you with your search for a small flat, we do not deal with student accomodation. I can suggest these links though:
All I can suggest is looking on www.leboncoin.fr for private rentals or www. airbnb.fr , and also www.colocation.fr
residences-etudiantes/ montpellier/34/studea- montpellier-centre2,309632. html
com/2,les-quartiers-latins, montpellier.php?PHPSESSID= 40806khp801lq3knpealkjr8o4
also try visiting the CORUM, they currently have a centre for student accomodation, there may be a list of flats available.