How to remove old System Administrator from Mac

Hi,

I purchased a refurbished MacBook air but have been seeing notifications that a public school wants access to manually control my laptop. Every time I click "deny" rather than "allow," I still get the notification. Is there a way to remove this public school from my laptop so that I will no longer see this notification? I tried going to Users & Groups - it says that I am the only current user, but there is a gray Guest User that says "Enabled, Managed" who I am not able to click on. Is there somewhere else that I should be looking to help address this?