Generally, the HM retained the post(s) ( in case of addl. vacancy) showing students enrollment and submit required documents of the employee, resolution etc. to the DIS (SE) to retain the post(s). After retention, your approval no. will be changed. If any HM fails to do it, and he releases the same teacher, that vacancy should be deleted. I want to say, this school will loss a post. But you have released after 4 years. So, you should inform to your previous DIS about the matter and keep a received copy for further (future) references. Thank you.