Per spotify support,
You now have to cancel your student account completely, and re-sign up, and not activate Hulu...and on top of that...you have to wait for your account to 'revert' to free, in my case, that's 10 days from now.
FWIW - I had this issue as well and hadn't found this solution until after I contacted Spotify support through their chat. They were able to update my account to free immediately so I didn't have to wait for the already paid period to expire and then you resign up. I'm not sure if you'd be credited if you actually wanted to keep the premium so you wouldn't have to pay again, but since I'd already decided to give it up, that wasn't an issue for me. Just figured I'd provide this alternative in case folks are impatient like me.