Department spokesman Sonny Jackson said Tuesday that two backpacks were also recovered containing items the teens were apparently attempting to steal. Police were worried they might have contained explosives.
Jackson said a lot of BB guns look very realistic and authorities don't want anyone getting hurt.
The school reopened Tuesday after police searched the building.
Police were called after a cleaning crew spotted the teens breaking into KIPP Montbello College Prep Middle School. They were arrested following a three-hour standoff during which police tried to coax them out using the school intercom system.