Rugby-mad 15-year-old Harry Case was left stunned when England fly-half Danny Cipriani interrupted a training session at Isleworth & Syon School to make the announcement.
Harry, who was born in Isleworth and plays for Ealing RFC, was nominated for the honour by his teachers due to his outstanding work and contribution to the school's sporting success.
He will fly the flag for England when they face Fiji at Twickenham Stadium in the tournament opener on September 18.
Harry is the first official Emirates flag bearer to be announced by the tournament sponsor.
Young rugby fans across the country now have the chance to join him in leading a team out at the Rugby World Cup by finding Emirates flags hidden in host cities around England and Wales, and posting flag selfies with the hashtag #EmiratesFlag.