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
Choir wins public vote, while judges pick dancers ahead of 12-year-old singer-songwriter Henry Gallagher.
The hunt for murder suspect Jed Allen has ended after police found his body two days after discovering his mother, stepdad and sister dead.
A woman who had been searching for her mother for more than 10 years has been reunited with her after posting an appeal on social media.