I found the same connection, happened 7 days ago, whilst my account is years old. I have never - ever - registered myself with SchoolKeep, nor I had heard of it until 5 minutes ago.
I found about it because somebody entered my Airbnb profile and used it to publish an highly priced, immediate booking, hosting listing, connected with a paypal account.
The dude didn't change my password, though, which makes me think they hadn't gotten hold of it in the first place - or else they would have used it to change it and kick me out of my account.
What I suspect, thus, is that somebody found a way to break into Airbnb by other means, probably through SchoolKeep itself.
Please, watch for "strange" entries in the recent account logins list of your account, which you can find in the security section of your account settings. Mine shows two suspect accesses.