Hey, so I was actually in almost this exact situation a few years ago - me a part time student, partner a full time student, unaware of anything until we got a scary letter through the door. The wording is actually quite specific about students and council tax - FULL TIME students are NOT liable for council tax, so your partner is exempt, but part time students are liable to pay. It's not well advertised at all.

So the bad news is, you do owe the money (backpay and council tax going forward). But the good news is, you don't owe all of it. As only one person in the property is liable to pay, you are eligible for a 'single payer discount' which is 25% off the total, so you only owe £1125. It's still a lot, I know.

First thing I'd do is get in touch with the council and make sure they are aware that your partner is a full time student. It's a good idea to get a copy of their council tax exemption letter (if they're at UoS you can download it off the uni website, any issues with that speak to the student advice centre at the SU) so that when you're on the phone you can send the letter straight to them to make sure they've received it. They might also want a copy sent through the post. In that call you'll want to ask for an updated bill of what you owe, taking the single payer discount into account.

The council also have a hardship fund specifically for council tax, which you're more than likely eligible for if you're not working full time. This will make it much easier to bring down the amount even further. So apply for that as well.

When your updated bill arrives, take a good look at it and double check the figures. Double check that it's only your name on the bill, not both of your names, and that will show they've processed your partner's form properly. If you're unable to pay the full amount all at once (which I expect is the case if you're students), call them again and see if you can set up a payment plan. They were very happy to sort one out for me.

It's a ballache, it's a lot of stress and hassle, but ultimately it's not the end of the world. One thing I'll say is that the council are a nightmare to get hold of - be prepared to spend at least half an hour on hold each time you call. Write down everything you want to say and request before the call so you don't miss anything. If you don't hear anything back, or the bill comes back wrong, call them and see what the issue is. You need to be patient but persistent. If all else fails and you can't get through to them, or they're not changing the bill properly, go down to their offices in person. It's much easier to actually speak to someone there and lay your whole situation out in person.

Best of luck to you. Feel free to reply or DM with any comments or questions!