Calverton Elementary/Middle was briefly locked down Thursday morning before police found a semi-automatic handgun inside the building, officials said.
City school police officials said a student was believed to have the handgun, which officers found inside the school, located on 1100 Whitmore Ave., shortly after it was placed on lockdown.
The lockdown was lifted, and one student is being detained in the incident, city school officials said.
"Providing safe, secure, and positive climates for teaching and learning is City Schools' top priority," city school officials said in a statement.