Why do people (who are not traffic guards or officers) think they have the right to stop traffic?

I live close to a school. The speed limit on the road is 40 mph. It is becoming a common occurrence that I would be driving down the road and all of a sudden a parent or teacher would walk out into the road and throw up their arms like they are Jesus and force all of the cars to stop. They would then instruct the children or who ever to cross the road. Are people allowed to do this? They are not crossing guards or police officers. It also seems like a very dangerous practice because cars will be traveling 40 MPH and then all of a sudden need to randomly stop.