many Pagans are in the broom-closet, which could be reason one. reason two would be prejudice, how long do you think it would take close minded individuals to picket, or vandalize, the school? third, most [not sure if all] Pagan religions, Wicca especially, don't want you pushing your faith on another, including your children. they have to learn themselves. sure, you could raise them to celebrate the sabbats, and if they ask a magick related question you can explain it, but ultimately, you cannot teach your child your faith, they need to choose.
i went to a christian school as a kid, a mandatory class every day was religion, where the Anglicans went to one room, the Catholics in another, and we studied the bible. one week they choose to teach us of other faiths, but wound up telling us all faiths are evil and wrong. while i don't think a Pagan school would do that, a child cannot choose yet if they follow a certain path. public school might be your best bet in this case since church isn't aloud in the school, unless the particular child chooses to pray or worship.
Mind you, a university, while it would still deal with prejudice and vandalism, would make more sense since they're adults who have chosen their faith. my college had it's own prayer room on campus. i went there a couple times, the place had people of many different faiths, it was a quiet room for us to go and worship if we had to for whatever reason. that i'm fine with, was never bothered there.