A bus driver took youngsters to school today unaware one of his windows had been "kicked out" – until children started climbing out of the hole.
Staff at Chalkwell Coaches in Sittingbourne had to study CCTV footage to find out how the glass came out.
Roland Eglinton, managing director of Chalkwell, confirmed it was an act of vandalism.
He said: "The CCTV has confirmed that the window was kicked out by pupils from Westlands School and we are working with the school to identify the culprits.
"The lower deck was full at the time and the driver would not have had a clear view of the rear of the saloon.
"He was, after all, at the front which is around 40ft away, with a bus full of passengers between him and the event.
"Indeed, he was not aware anything had even happened until he got to the school and the same pupils climbed out of the window.
"Our vehicles are maintained to high standards and we are independently audited on a regular basis.
"There was no fault found with the vehicle, which has since been repaired and put back into service.
"Once identified the pupils will be referred to the police."
The vehicle was immediately taken back to its depot in Church Road, Murston, and another driver despatched to retrace its route to track down the glass panel.
The window was found undamaged in Grovehurst Road about 15 minutes from the secondary school.
The packed 321 had been travelling from Iwade to Sittingbourne as part of the morning's school run.
Chalkwell has been operating since 1931 and is in its third generation of family management.