The boy, whose name was not released, brought the toy gun to Elmwood Place Elementary School Monday and threatened classmates with it, The Cincinnati Inquirer reported.
At the end of the school day, school administrators alerted police that the boy had shown about six students a gun, police officer Leah Schindler said Tuesday.
"All of the other kids thought it was real," Schindler said, adding that at least one student reported the boy had threatened to fire it.
When questioned by police, the boy said he brought the BB gun to school because other students, including older boys, made fun of him because he wears braces on his ankles and because he is small for his age.
A year ago, "some teenagers threw him into a garbage can," Schindler said the boy had told her.
The boy is facing a delinquency count of inducing panic for the incident, Schindler said.
School administrators have not commented on the situation.