How to reset a salvaged Mac Pro which has previously been managed?
That machine is too old to boot to internet recovery. You will need to bring another OS - perhaps installed on an external hard drive and hold the option key when you boot. A DVD works as well, you will likely be able to copy files off but will need some skills to bypass deep freeze, that’s designed to keep out highly motivated teens with hours of free time. 

The easy path is to erase the drive and install the OS you please. 


https://support.apple.com/en-us/HT201255


The help above shows the various startup keys and links to other articles on reinstalling. 

Eventually, your path will land at the bottom of this article covering other ways to reinstall if you can’t get past deep freeze. 


https://support.apple.com/en-us/HT204904


Since you have an external, just use disk utility to wipe and then clone it or rerun the installer once you have backed up any files you want.