(2-part post):
The landlord of 'multiple occupation' properties is legally responsible for paying the Council Tax, rather than individual tenants. However, he's entitled to pass the bill on.
Council tax is payable in full on a property where there are two or more 'qualifying adults' resident at the address. Where only one qualifying adult lives at the address, a 25% reduction is applied. Full-time students do not count as 'qualifying adults', so the bill will now have increased from zero to 75% of the full amount. The landlord must pay this but he can demand that the tenants refund the money to him.
The exact way that the bill should be split among the tenants should be defined by the agreement which exists between each of them, and the agreement which exists between them and the landlord. Regrettably, the arrangements in relation to shared property are frequently not written down in a formal contract. However, if one member of the group is responsible for collecting the rent from everyone, and forwarding it to the landlord, it's likely that the same person will be legally responsible for ensuring that the Council Tax is paid to the landlord.