Bullet holes were found on two school buses in a district near a series of highway shootings.
Thursday's classes were canceled. They may be called off again Friday.
All 200 buses in the district are being checked. There are about 20,500 students in kindergarten through 12th grade in the district.
A woman passenger was killed in a shooting last month. No one else has been hurt. Police believe there have been 16 shootings since May.