Hello, welcome to Ridgeline Academy. You're dead. Has that sunken in yet?
Yes, you're dead. You're dead, but you're going to school. This isn't heaven or hell, and in a way this school is stuck between the two places. You might say it's almost purgatory but nicer. You'll still need to eat and take care of yourself and you can die again, though human methods won't stick. And lucky you, you can even pick between being a guardian angel or a reaper.
A guardian angel does exactly that - protects those who they've been assigned to - and a reaper does the dirty work. They reap the souls of the living and if someone is putting up too much of a fight against death they can deliver the final blow to hurry them along.
Your job is to go to school and learn these trades. What happens in the end? Well you'll just have to find out.