Computer is still managed by organisation after I left

This is also an issue I asked Chrome support, but there are some stuff for Apple. When I joined my school they set up my MacBook Air, and everything was fine. After a few years, I left the school, and still use the MacBook (as it is mine) and they claimed to have unenrolled my computer from the system. But Chromes' home button still takes me to the school website, and my computer still says "Managed by ​​[schools' name]", and I don't know what to do. ​I am the admin of my computer, but there is another profile on my computer that while I have removed its admin, I can't remove the account, and I believe that that is the IT expert of my school. I have tried deleting the account but I need its password (which I don't have) and whenever I try to change the password I get an error message "Reset password failed." With an icon of two people's silhouettes with a blue background (the users and groups Icon on system settings). If I try to install any Chrome extensions it says "Blocked by admin" as my school didn't allow Chrome extensions, and while my profile should be the only Admin on my computer, I don't know how to change that. at my school, we also weren't allowed to change our passwords, and when I try now it says "The server is not available. Change your password when the server becomes available." I'm not sure if that is just the server having a problem or if that's because of the school, I never tried changing my password when I went to that school, so I don't know if that's the error message that'd show up. I also can't change this other user's settings other than "Allow this user to administrate this computer." I think that's it though. I want to add the reason a lot of things apply to Chrome, is I copied and pasted this to Chrome support as well. I left the school about 6 months ago, and it was my computer they set up, not a school computer