I don't know exactly what the criteria is for Centennial - my district - but I know for where I grew up in the Mounds View School District, which begins 4 miles from me, the only criteria was could the buses get through to pick up the kids. If the buses could get through, then school was on.
That being said, MV didn't have walker zones like Cent. does. We have to walk if you live within a mile of your school and you don't have to cross a major roadway to get there.
I still think it's plain pride that they are able to stay open. Really, what kid is going to have their mind on their schoolwork anyway with the first really BIG snow of the winter? Mine aren't! Granted they are Kindergarten and 1st grade, but still! I don't think it's smart of them at all.
ESPECIALLY as Forest Lake 10 miles north of us closed for the day and Spring Lake Park, the district next to ours is going late start.
Personally, while I would have loved the school to close, I think a late start would have been more appropriate. It would have given the plows a chance to do a good job, given the salt a chance to melt some of the ice that was underneath the snow and maybe kept some people a little safer.