The Teachers purpose is to educate prisoners.
Teachers are external freelancers who will come to your Prison for the sole purpose of teaching your prisoners new skills. They are hired externally unlike other staff members.
Teachers will only arrive at your Prison when you have scheduled an education program, i.e. the Foundation Education Program or the General Education Qualification. They will arrive just before the first scheduled lesson and will leave when the last lesson finishes.
Teachers can be unlocked through Bureaucracy but cannot be hired like other Staff Members. They are hired externally at no cost, but the education programs you choose to run do cost money.