John Horn High School in Mesquite was locked down Thursday after one student said another had a gun in his backpack.
Police learned that one student had threatened another and partially displayed what appeared to be a gun.
The student, his bag and his locker were searched, but no gun was found.
The school was being kept on lock down while a thorough search was conducted, during which the students did not change classrooms but were allowed to use the restroom or have lunch in small groups.
"Though there was an exhaustive search, no gun was found. We commend our students for handling this situation in a calm manner, and thank our parents for their understanding," said Elizabeth Fernandez, director of communications and marketing for the Mesquite Independent School District.
The student alleged to have the weapon was taken into custody by police.
