Decide whether or not the roads are safe enough for buses to pick up and drop off students safely. This takes a complete understanding of the roads of your county. If there are many back roads, mountainous features or other questionable terrain, assume those roads will be a slippery hazard if there's ice or snow.
Check the temperature and precipitation conditions. Can walkers safely make it to school and back home? Freezing rain, slippery sidewalks and extremely cold temperatures make it unsafe for children to walk distances outside.
Think about the parking lot at the schools. Will they be a hazard to faculty, staff and students? If the temperature is at or below freezing and precipitation is falling, it's a safe bet that the lot will be dangerous.
Call supervisors in surrounding counties. Collaborate with your peers to come to a decision. If surrounding counties are closing schools, it's likely wise to close schools in your county as well.
Watch weather reports to get an idea of how long the hazardous conditions are supposed to last. Call off days ahead of time if it's clear the foul weather is supposed to last. This gives parents the chance to arrange for child care.