Yes this is the procedure of going into rescue. This gives you root access, now anything can be done. This is a very simple technique.
One of the reason why majority of the servers are kept away in such a place so as no one can reach them. Also, having a grub password helps in this case. Maybe you can suggest this to your teacher as a solution.
Another method could have been throught sudo. If you can access root shell as sudo, then it's possible to change the root password, unless it's been configured so.
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Forgot to add, it is known as single user mode, that's why the parameter single was passed.