A short sale with a tenant can be tricky. Hopefully your agent asked about the current owners lease with the tenants. The lease will superceed the contract. What that means is if the tenants have time remaining on their lease, you the new owner cant move into your home until the lease expires and they vacate. The current owners can contact their tenants and ask if they would like to break the lease, but the tenants are in control. Since the current owners are usually not paying the mortgage perhaps they can offer sine "rent free" or "reduced rent" time as an incentive to break the lease.
If the renters are currently month to month then you will want to make sure that your short sale approval letter from the bank gives enough time for the tenants to be properly notified to vacate. If you are buying your home to occupy yourself, your bank is going to demand that you move in at the time of closing.
Your offer to purchase will have a section for renters and how they will be handled prior to closing.