There were kids in our 4th parade on dirtbikes, quads, and John Deere Gators (and clones) without helmets. Along with bikes without helmets. Riding a bike in a parade, at parade speeds is not dangerous and does not need a helmet.
As far as the teacher not wearing a helmet, I do some substitute work at an elementary school and I never wear a helmet when commuting there. Bicycling to the school is not dangerous and does not require a helmet.
When I mountain bike, or fast road bike, I wear a helmet.