Factory reset on Mac

Hi! I have a question. I had the opportunity to take home a Mac desktop with macOS Mojave from my school since the district has switched over to windows. I’m trying to do a factory reset so I can start fresh with it. However, after many attempts I can’t erase the current OS. Even with trying to go through recovery mode, all I get is a Lock Screen. My IT guy at school can’t figure out why my password won’t work and doesn’t know what else it could be. Is there any other way to bypass this Lock Screen without actually bringing the device to Apple for support? Any help is greatly appreciated!