It is for the returning officer in each are to decide which public buildings will be used as polling stations in that area. The schools, and more particularly, the teachers, have no say in it. In this area we use a mix of schools and other buildings. I think the reason so many schools are used generally, is that they are public buildings which are usually within easy reach of most of the residents of an area.
The scoots are usually closed because: a) There is a security aspect relating to the pupils. The doors to a polling station cannot be locked and therefore the building will be accessible to anyone. Most school buildings, especially if catering for a large number of contituents, will use the hall for polling and this cannot usually be easily sealed off from the rest of the school. b) as somebody else mentioned, there is the security of the ballot box to be considered. A couple of years ago, we had an INSET day on polling day and we used a completely different part of the school to that which was being used for polling. The polling officer complained to the returning officer because they are supposed to have exclusive use of the building and the headteacher received a telling off and instructions that under no circumstance were the staff to be in school on days when the school was fulfilling its obligation to act as a polling station.
It is not always possible to re-arrange INSET days at short notice, Good speakers are usually booked well in advance and are not usually able to switch to a different date to suit a school.
I am sure that most teachers are not especially aware of all the reasons why their schools close. They are just told that it will.
I am always amazed at how so many people on here seem to regard schools as babysitters for their children! (Mind, at an average of £5 per hour per child, maybe teachers should consider charging childminder rates. They would earn an awful lot more than they do at the moment. )