A new map from Reddit user atrubetskoy is sure to stoke this regional competition. Using data "taken from hundreds of various points from user responses...interpolated using NOAA's average annual snowfall days map," Trubetskoy made a map showing how much snow it typically takes to close schools in the U.S. and Canada. Notice that for much of the southern U.S., all it takes is "any snow" to shut schools down. For the Upper Midwest and Canada, two feet of snow are required for a closure.
In much of the Midwest and Great Plains, school closing often depends more on wind chill and temperature than on snow accumulation ("cold days"). Thus, this map may be misleading in those areas.
Many jurisdictions in California and other western states have significantly varied snowfall, depending on elevation. This makes it difficult to find an "average" number, or often makes it misleading.
Urban areas like Chicago and New York have more resources to clear snow and often need more to cause closings.
Clarification: The lightest green says "any snow" but also includes merely the prediction of snow.
Clarification II: This is snow accumulation over 24 hours/overnight.
Hawaii does get snow! Just... not where people live.