I think it's relative. For example, here on the south shore of Boston, our schools do NOT close for 6" of snow. They don't close for 10" of snow! In fact, my DSs have had school with close to 2 feet of snow on the ground. In some states that are not used to dealing with snow, they close after the 10th snow flake! LOL However, THESE temperatures are NOT the norm, even for New England. These are record-breaking temperatures. As one principal on the news said "Children do not have sub-zero clothing and accessories, in order to stay warm while at the bus stop, or walking to school, in these kinds of temperatures". He's right!! Even all bundled up in parkas, hats, coats, etc. that's just not enough. Also, most school buses use diesel fuel which coagulates and would make the buses difficult, if not impossible, to start up.
So MY opinion is that the schools did the right thing in closing the schools for today (Friday). I had already decided that if the schools weren't going to be closed today, then DH would drop the boys off on his way to work.