Lighting ballasts regulate the current to the lamps in fluorescent lights and provide sufficient voltage to start the lamps. Prior to 1979, PCBs were commonly used as an insulator in ballasts. In 1979, the USEPA banned the processing or use of PCBs, except in totally enclosed equipment. However, a large number of fluorescent light ballasts that were installed prior to the ban or that were stored and used after the 1979 phase-out, may contain PCBs and may still be in use in the U.S.
The most likely way that people are exposed to PCBs from the ballasts is through breathing PCB-contaminated air or, if the ballast ruptures, by touching PCB-contaminated materials. When they remain in place, leaking ballasts can continue to release PCBs over several years and create elevated levels of PCBs in the air that students, teachers and other school workers breathe. The USEPA recommends removing PCB-containing ballasts from buildings as soon as possible to prevent exposure.
USEPA guidance (schools) is here.