Three children were taken to hospital with facial injuries after a rock was thrown through a school bus window.
The incident happened on Thursday on the A133 near Frating Green between Colchester and Clacton, Essex.
The missile was thrown as a double decker bus was taking children home from school shortly before 1630 GMT.
A 12-year-old girl and two 13-year-old girls were treated at Colchester General Hospital for injuries believed to be caused by glass splinters.
An emergency paramedic and an ambulance treated children at the scene.
Ambulance technician Rob Knights said: "The rock went through a side window on the lower level of the bus.
"Fortunately the girl hit by the rock was not hurt badly and was not knocked unconscious.
"The three girls were extremely upset by what happened. There were at least 20 youngsters on the ground level with more upstairs. Everyone on the bus was quite shocked.
"There was every chance that this could have led to serious injuries for one or more of the pupils on the bus. It was an absolutely reckless act."
A 13-year-old girl from Tendring was treated in hospital, along with a 13-year-old from Kirby Cross and a 12-year-old from Frinton.