A total of 20 children and 10 adults were on board the 71-seater coach when a nine-year-old boy fell from the vehicle on the A47 near King's Lynn this afternoon
The youngsters are from Stamford Rugby Club in Lincolnshire and were travelling home after playing in a tournament at Holt in north Norfolk when the incident happened.
Superintendent Dave Marshall said:
This is a serious incident which has left a nine-year-old boy fighting for his life.
The circumstances of how the boy came to fall from the coach will form part of our investigation and it would be inappropriate to speculate further at this stage.
The boy's parents, who were travelling with the teams, are being supported by specially trained officers.
More top news
Murder charges against an Irish nanny, who spent more than two years in prison accused of killing a baby in the USA, have been dropped.
Charities face probe after claims they may have been involved in buying and selling widower's details.
The island of Kivalina in Alaska is quickly disappearing into the ocean, watched by its 400 residents, due to climate change.