I have seen some schools in Vancouver with "Peanut Free" posters posted around the school. My assumption would be that those from the local Coastal Health Unit. There is a binder for Coastal Health info in every school, and thee likely is info in there.
Not all parents come into the school and see posters. Some never read the newsletters that are sent home with notices that there are students with severe allergies
Kindergarten is sometimes easier to manage if the kindergarten does parent meetings at the beginning of the year (which many do) If the teacher knows ahead of time, she can tell parents when they come for meetings.
The kindergarten child at my previous school had such a severe allergy even traces of peanut butter on the wall could trigger a reaction. The school nurse gave a presentation to all the staff about severe nut allergies, using an epipen