Hargreaves attended Wharton High School in Tampa, Florida, where he played football. As a sophomore in 2010, he had 44 tackles, two sacks, and two interceptions. As a junior in 2011, he had 49 tackles and two interceptions, along with 11 offensive touchdowns. As a senior in 2012, he had 110 tackles and five interceptions. He was recognized as the most valuable player (MVP) of the 2013 Under Armour All-America Game.