When the weapon wasn't located, some classes were canceled and others were held at different locations. The campus houses several different schools.
Officials say the teens aren't students at the school and there was "minor damage" to the building.
Classes were cancelled Monday at the DuSable Leadership Academy of Betty Shabazz International Charter School.
Students who attend Bronzeville Scholastic Institute and the Daniel Hale Williams Prep had school Monday. Their classes were held at the Vacant Farren School building.
Chicago police officers are investigating the incident.
The Associated Press contributed to this report. All rights reserved.