A head teacher has spoken of his shock at seeing a video clip posted on a public website of a laughing pupil hurling a rock at a classroom window.
The boy was laughing as he was filmed throwing the rock
The shaky 15-second footage shows a clearly identifiable boy grinning as he strides up to throw the missile.
Head teacher Gordon Cunningham said it had been the Year 9 pupil's last day at Easthampstead Park School in Berkshire before he and his family emigrated.
"It's horrendous," he said. The police would be informed.
The clip, featured on a popular video-sharing website, also shows a boy and a girl dressed in school uniform who appear to have been encouraging the attack, while other voices can also be heard.
'Teacher deserved it'
Shouts can be heard of: "Everybody ready? Right come on, here we go," as the boy takes a run up towards the window.
The rock or lump of concrete that he throws smashes the glass - although the window does not break completely - and the clip ends with the group running away and laughing.
The video was apparently made at the end of last term
On the website, the video is accompanied by an explanation from the perpetrator saying his teacher deserved it for the way he had been treated all year.
Alerted to it by BBC News, Mr Cunningham said: "You can see from the video this is an act of wanton violence.
"It's not just the audacity of it, but to video it and then put it on a public website...."
He said it appeared to have been a parting shot at the end of the boy's last day at school, before his family moved to Canada.
'Fifteen minutes of fame'
The school said it would be in contact with the family. The other identifiable students had already been disciplined.
Chris Keates, general secretary of the NASUWT teachers' union, said: "Such behaviour is completely unacceptable and could have resulted in injury to staff and pupils as well as to the property.
"Unfortunately, any yob or vandal can now have their 15 minutes of fame, aided and abetted by readily accessible technology and irresponsible internet sites which enable such behaviour to be glorified."
She said the union supported a zero tolerance approach in schools to pupils who used technology to abuse and undermine teachers, and called for more rigorous legislative control of internet sites which gave them licence.