They're saying that the 4°F weather that we're having (-20° with wind chill) is dangerous and therefore, they've closed over 500 schools. (Dude, that's a lot of schools. Seriously.)
But then they had to remind parents that they really shouldn't allow their kids to go sledding, or really, play outside, in this weather. If it's too cold to wait for the bus, it's probably too cold for snowball fights.