Hm, maybe I forgot something essential I did: I setup virt-manager to use virtual machines (probably the program “boxen” can be used also).
By doing so I can have a Debian and PureOS system up and running in a virtual machine. After installing a virtual machine you can take a snapshot that conserves the actual state. If you brake the system on your virtual machine by trying things out you can just hop back in time to that snapshot.
It is really a computer inside your computer and by design you cannot break your host system (the software running directly on your hardware - well, if you’d find a way you could give a talk at some security conference 
).
Edit: Thinking about it, I’d say that using a virtual machine for training using a remote connection and VoIP could be an idea. There wouldn’t need to be any user data in that virtual machine (wherever it resides) and nothing seriously needed could be broken. Trust would also be much less of a problem than with a direct connection in a production system.