It is fully depends on the basic needs of the school.
Suppose, you are an HM of an institution.
1) You joined and see the tenure of M.C. is going to expire. If there is none to help you, then to complete the election you need minimum 20/25 on duty.
2) Then you find a staff is going to retire. His paper works and submission .
3) You find your school has no roster. So there is vacancies 4/5/6/7 or 8 for last few years , whose P.P.O. will not get till the Roster Regularized and authenticated. You have gone to Writers’Building ( 7/8 times).
4) You yourself make and submit the I.T for your staff and submit 3 months interval.(5/6times)
5) You find you should upgrade your school to class XII.
6) D.I./A.I./WBBSE/WBCHSE/ Meeting of SSM, D.I./A.I./,S.I./B.D.O./S.D.O./BCW/DMO/ others, others (thousands) etc.
*** Dear friend, If you want to act as a perfect and real H.M. then you will not get time to spend with your family, mainly with your child.A lot of work load will come before you.
Of course, you may ignore those to get your only salary but you will not get the eternal rewards from the men.