2010-11: Miles Gendron skated for the CM Outlaws in the Massachusetts Midget Select League in the fall before playing for Rivers School in New England prep hockey. He scored 7 goals with 5 assists in 15 games with the Outlaws. In 24 games for Rivers, coached by former NHLer Shawn McEachern, he scored 2 goals with 6 assists in his freshman season.
2011-12: Gendron played in 24 games for Rivers School in his sophomore season. He scored 7 goals with 9 assists. The Red Wings missed the NEPSHIHA tournament; finishing 15-12-1 and ranked 35th amongst New England prep schools.
2012-13: Gendron skated for Rivers School and was invited to USA Hockey's Select 17 Festival. Converted from forward to defense by McEachern in mid-season, he scored 12 goals with 16 assists in 28 games. Rivers was seeded second heading into the Piatelli/Simmons Small School tournament but fell to 7th-seed Holderness, 3-2, in overtime. Gendron was selected by Moncton in the 10th round (191st overall) in the 2013 QMJHL Entry Draft and by Lincoln in the 13th round (191st) of the 2013 USHL Entry Draft.
2013-14: Gendron, converted to defense full-time, skated for the Neponset Valley River Rats U18 team and received the John Carlson award as the top senior high school player in eastern Massachusetts for his play at Rivers. In September he committed to playing college hockey at the University of Connecticut in 2015-16. He scored 1 goal with 8 assists and 9 penalty minutes in 11 games for the River Rats. A team captain at Rivers, he scored 6 goals with 13 assists in 22 games as the Red Wings captured the Eberhart Division championship. Gendron was ranked 74th amongst North American prospects in Central Scouting's final rankings prior to the 2014 NHL Draft.