Can I add a backup server to my single server UCS@school environment?
No, you can not. A single server environment is just that - a single master.
If you want to use a backup and/or other systems you have to start by installing the DC master as a multi server environment and join your school DC (a UCS slave) to the master. Now you can add additional systems like a central backup, a DMZ slave for services,…
The manual also describes how to convert existing single server environments into multi server environments: