Short term rentals

Tenancy Types and Management

I have just refurbed a property which I have split into small one bed flats. I have recently advertised them and as the property is near a large hospital there is a lot of interest from young people who seem to only require shorter term accommodation whilst they are training or contracted to the hospital. They seem to be looking for 2-3 month periods. The attraction is that they would rather spend the time in a flat with all facilities than stay in a hotel/guest house. It certainly would be cheaper even though I can ask a higher rent than I would for a longer term tenant.
I would appreciate any advise on how I can approach this as an AST does not seem suitable for this type of arrangement. Is there any other form of agreement that caters for shorter periods than AST's ?