Clayton, N.C. — Students complained of headaches after their bus hit a rock on Monday afternoon, Johnston County school officials said.

A school bus driver stopped the bus at Santa Gertrudis Street in Clayton after hearing a noise she believed was the bus running over a rock, said Crystal Roberts, the school system's public information officer. Thirty students were riding in the bus at the time.

Some students got off the bus when the driver tried to check for damage, despite the driver's orders to stay aboard, Roberts said.

When the students got back on the bus, a handful started calling their parents, saying that their heads hurt. The complaints forced the driver to call for emergency medical personnel, Roberts said.

Fifteen of the students complained of headaches, and EMS workers had to check out all 30 of them.