Every HM/TIC receives such order, generally from respective DM/SDO, before conducting any Competitive Exam in the institution stating that every employee under his/her control is bound to perform invigilation or any other duties as assigned to him/her. Not only that, even if your school has not been selected venue but you got appointment as invigilator from the HM of a nearby school/ DIs/ AIs/ SI/ BDO, you are bound to perform the duty.
Here I attach one for reference from the Secretary, Govt. of West Bengal