The cold overnight temperatures in the Ann Arbor area are illustrated nicely by the Weather Underground's Wundermap for the area - here's a snapshot from 12:30 a.m. Brrr!
Assuming that school is on tomorrow (and not called off because of bitter cold), this should be one of the few days of the year I don't just send my second grader off to make it to school on his own.
If the Ann Arbor Public Schools were to be called off because of bitter cold, it would be following these guidelines: no outdoor recess if temperature or wind chill is below zero, and no school if temperature or wind chill is below 20 below zero. Their table, under "Unsafe Weather Guidelines":
WDIV, Channel 4, "Click on Detroit": Detroit School Closings. (11 reported at 1 am.)
CBS Detroit, WWJ 950 AM: First School Closings. (11 reported at 1 am.)
Fox 2 Detroit: School Closings (11 reported at 1 am.)