A teacher who receives a pension shall not be re-employed in the Teaching Service on contract or otherwise unless it is established to the satisfaction of the Commission—
(a) that the pensioner has a range of experience and technical qualifications, which make him or her particularly useful to the Teaching Service;
(b) that it is not possible to fill the particular post by promotion of a suitable teacher or by a new appointment; or
(c) that the appointment of the pensioner would facilitate and is necessary for the training of a successor.