I find that not all students leave over Christmas.
Often the students that do, return. Normally I keep rooms for them for one month. If they want to stay longer overseas and want a room they must pay before they go. I also keep their bond and one weeks rent in advance. Overall I would allow a vacancy rate of about one month per tenant per year to be safe.

I try to aim at renting the rooms for 2 semesters at a time and never for shorter than 1.

Another good tip is to make sure that the people from different cultures are compatable. Many different Asian cultures in a house can be a big mistake.

Have a look at the Rooming House Residential Tenancies Regulations in your state. You can print it off the internet together with a tennants agreement form.

Cheers,
John