It really depends on the school as well. In the "big snow" winters of 2009/2010 and 2010/2011, our school only closed on 1 day and opened late 2 mornings; whereas most other schools locally closed for over a week. They had to cancel summer mid-term break to make up the days.
Our school, after the first year (when people just volunteered), set up a proper "snow team" system. A parent is coordinator, and gets a message from principal or caretaker. That parent lets all the other parents involved know. As many of those other parents on the team who can make it in, go to the school for 7-7.30am. Caretaker and coordinator both have keys (and principal tries to get there too) and open up shed to hand out shovels and brushes.
The parents job is to clear a path from the gate to the school door, a path around the school, and space in the yard for the usual "lines" to form (if not too onerous, also some playing space in yard).
The intention is that the school can open either on time, or with a 15-30 minute delay (which principal will notify on the text message system to all parents). But remain open as many days as possible once the building itself is ok (heating and plumbing not affected).
We haven't had to use it yet, since it was actually formalized, but we are ready to spring into action if needed. (We don't get a lot of snow here).