My kids went to montessori schools and, like most preschool kids, they played outside even with snow on the ground. In winter months when there was snow, we sent them with their regular pants (jeans or sweat pants) and gave them snow boots. We also put regular sneakers in their backpacks for when they got back into the classroom. We also sent hats and gloves.
I NEVER sent my kids to school with snow pants. If it's that heavy a snow that they need snow pants, either school will be closed or they won't be outside. You should send extra clothes to be stored by the teacher so in case your child gets wet.
The only time we use snow pants is when we go outside to sled or when the kids go run through the snow on weekends (or snow days). I think last year, we used them 3 times (we don't ski). I bought snow pants at Land's End. (www.landsend.com)
You can also get them on gap.com or llbean.com or any ski store.