That decision is left up to the principle and/or the superintendent. In some areas, a few inches would be enough and some areas do not cancel school for snow at all.
whatever you want really. it's best to have powder that is packed down and covers the grass, dirt, etc, but as long as your skiis will go down the hill, then it's enough
but, the more snow, the...
It depends where and what level. Local Police usually just require a high school diploma and then attendance at a police academy. Major cities local law enforcement such as Chicago, New York, etc...
You need enough to cover any rocks and other protrusions, but also enough that you won't go through the snow with your weight. Otherwise, you'll be gouging out the base, and that's no bueno!