I worked in student housing for a number of years, primarily as a manager of a 780 bed property (a mix of 2, 3,and 4 bd Apts, all rented by the room). Most of the key points have been touched already, renting by the room, furnished/unfurnished, guarantors and dealing with parents. But another thing that needs to be considered when renting by the room is roommate matching. Depending on the number of units that you have this may be something to consider.
Often times people will move to a new school without knowing anyone and can't afford to live by themselves. We would have tenants fill out a short "about me" questionnaire and we would attempt to match roommate based on similar interests. Sometimes this would work out, but other times it would blow up. There were many times that I had to act as mediator between roommates and help them work out their differences. Just something to consider if you rent by the room and have a group of 3, or 2 that show up to rent your 4 bedroom apt.