Council Tax Exemption

Hello, I've just moved into my uni accom and received an email stating if i don't prove i'm exempt from council tax i'll have to pay an extra £30 a week but when i go on the local council website it claims i can't even request an exemption unless i show my bill number (i've never received a bill from the council?). I'm stuck on what to do the accom's rent is already 100% of my student loan i can't afford the extra thirty so any advice would be very helpful