“We will not consider school cancellation for low temperatures until the temperatures get between -20 to -25 below zero (Fahrenheit) with the windchill factor included. Parents can always choose to make their own choices and do what they feel is in the best interest of their child(ren). Listed below is a chart that explains how the windchill factor can cause frostbite when it approaches cold temperatures.”
https://www.nws.noaa.gov/om/winter/windchill-images/windchillchart3.pdf