The elementary school I went to as a child had a fairly large playground area, but only the front third or so had any actual playground equipment on it. The teachers had a tough time monitering us if we wandered too far back, so they required us to stay within the closest two-thirds of the yard or so.
Behind the playground was a fenced-off, wooded area with a lot of trees. All of the students believed that the real reason to stay close to the building was because there was some kind of crazy hobo that lived in the woods and would kindnap children who wandered back too far. Anytime we saw a piece or trash (especially paper) on the playground, we would say that it was the remains of some kid he caught and burned to death.
At some point, the hobo changed into Pennywise the clown, which only strengthened our beliefs (it made more sense that an evil clown being would kidnaps kids than some hobo). I wonder if the kids who go to the school still believe that.
A childhood belief about monsters

others believed things like this
- My best friend used to believe there were leprachuns to dig up in the playground at recess— deanna
- I believed that our teachers lived at school. They were teachers--they had no reason to leave :)(Anon
- It never occured to me as a child that teachers lived in houses. I thought they lived at school...— Heidi, Pittsburgh, PA
- I used to believe that my school teachers lived inside the school and my school bus driver lived on the bus. Anon
- I used to think that a clown lived under my bed and tried to eat me while i was asleepAnon
- When I was in elementary school I use to think that teacher lived at school at their desks all year long.Anon