Understanding council tax bill when one is no longer a student

Spendless

Currently my DS houseshares with 2 other students. 1 housemate is on yr2 of their degree as is DS, the other is on their third and final year.
At the minute as they are all students they are exempt from council tax. My understanding is this will change when the 3rd housemate officially finishes their course and then the bill will go to 3rd housemate minus a 25% discount for a single person. Is this correct? I doubt 3rd housemate will be in a position to pay this bill as they don't have any work  so can they also apply for a council tax benefit? When are they deemed to stop being an official student, is it when the term ends? 

CIS

As long as all the parties have the same legal interest - i.e. all tenants - then the non-student becomes liable for the council tax charge on the property, with a 25% discount. They will be able to lodge a claim for council tax support.
The university will confirm when they cease being a student, the council will rarely argue with this date.