Freshman and sophomore seasons Giles attended
Wesleyan Christian Academy in
High Point, North Carolina, where he was coached by
Keith Gatlin. As a freshman, Giles averaged 12.5
points per game and 9.5
rebounds per game after leading Wesleyan Christian to a 2013 NCISAA 3A State Championship alongside future and former NBA player
Theo Pinson. Giles missed his entire sophomore year due to a left knee injury. During the 2014 summer, Giles would return from injury joining Team CP3 on the EYBL circuit. He also participated in the
Under Armour Elite 24 game in
Brooklyn, New York, finishing with 16 points and 11 rebounds and earning Co-MVP honors.
Junior season In his junior year, Giles and Wesleyan were ranked the No. 2 team in the Nation by USA Today. On November 14, 2014, In his second game back since his injury, Harry scored a career-high 38 points and grabbed 19 rebounds in an 82–58 win over
Northside Christian Academy. On December 21, Giles scored 29 points in a 67–62 win over
Mater Dei at the City of Palms Classic in Fort Myers, Fla., ending the team's 46-game win streak. Giles and the Trojans then played in the 2014–15 High School OT Holiday Invitational Tournament at
Needham B. Broughton High School in
Raleigh, North Carolina. On December 29, Giles and Wesleyan defeated
Word of God Christian Academy (98–85) behind Giles 31 points and 17 rebounds to advance to the championship game. On December 30, 2014, Giles went head to head against an
Orangeville Prep team that featured future NBA players
Jamal Murray and
Thon Maker. Wesleyan would go on to defeat Orangeville Prep (78–75) with Giles scoring 26 points and 14 rebounds while Maker scored 24 points and 11 between the two. On January 15, 2015, Giles scored 17 points, 12 rebounds, and 4 assist to help the Trojans defeat
Malik Monk and
Bentonville High School (63–55). On the season, Giles averaged 23.9
points per game, 12.5
rebounds per game, 2.0 assist per game, and 3.0 blocks per game while leading the Wesleyan Trojans to a (30–5) record and a NCISAA 3A state championship game appearance, losing to in-state rival
Greensboro Day School. At the end of his junior season, Giles earned first-team All-USA honors by
USA Today. During the summer of 2015, Giles would rejoin his
AAU Team, Team CP3 All-Stars, sponsored by fellow
Winston-Salem native and former NBA superstar
Chris Paul. Giles averaged 18.2 points per game and 12.0 rebounds per game in 16 games on the
Nike Elite Youth Basketball League circuit, earning first-team All-EYBL honors. In August 2015,
Slam magazine would name Giles to its Summer All-American Team.
Senior season Before his senior season, Giles decided to attend and play for the high school basketball powerhouse
Oak Hill Academy in
Mouth of Wilson, Virginia. Giles dominated the majority of his high school career; however, his senior year ended with a
torn ACL in his right knee. The injury occurred during his first scrimmage game with Oak Hill. Weeks later, Giles enrolled to (now-defunct) Forest Trail Academy in
Kernersville, North Carolina to take online courses to finish his senior year of high school while rehabbing from his knee injury. On November 6, 2015, Giles made his verbal commitment to attend
Duke University and play for the
Duke Blue Devils live on
ESPN joining fellow five-star 2016 recruits
Jayson Tatum,
Frank Jackson, and later
Marques Bolden. Giles chose Duke over other offers from
Kentucky,
Kansas and
Wake Forest. He was selected to play in the 2016
Jordan Brand Classic and
Nike Hoop Summit but was unable due to injury. Giles was rated as a five-star recruit and considered the best high school prospect of the 2016 class. Giles ranked as the No.1 overall recruit and No.1 power forward in the 2016 high school class by ESPN, while
Scout.com and
Rivals ranked him No. 2 in the Class of 2016 only behind
Josh Jackson. ==College career==