My wife is a retired teacher. She was trained in Secondary education teaching A and O level History and, after 15 years' left the profession. She then went back and went into Primary education, without having to be retrained. Later she went into Infant education, again without being retrained So it is possible to do it. However we both laughed at Brian becoming the Head of a Primary school after working as a Secondary school teacher. We think the other Primary school teachers would have a point of view about his appointment.
As Micmac implied this is "Soapland" so anything goes!