Another problem to throw in is the wonderful Ofsted and league tables - if the school is open, student attendance is poor - it flags up with the Local Authority that you have an attendance problem - Genius! Which head wants that on their record??
My school is currently shut due to the roads surrounding the school being too dangerous for the large number of our students who come to school by bus! Unfortunately health and safety play a huge part in this - one child slips on the ice/snow and the school get blamed, so you can understand caution. (Bear in mind an incident a few years ago where a student was doing PE on the astroturf in the rain, did a sliding challenge and tore his ligaments - who gets blamed...).
And as mentioned before, if staff can't get in then it does create a real problem for staff to student ratios. We often find that because infant and primary schools shut first (as their kids need much closer supervision) the teachers who are parents are then forced to stay at home to look after their kids, making the problem worse!
Still, hey-ho!