Born in Tulsa, Oklahoma, Ferguson moved to Dallas, Texas as a youth and enrolled at Prime Prep Academy. He joined the school’s basketball team and played for coach Ray Forsett.
As a freshman in 2012–13, Ferguson averaged 10 points per game and helped his team to a 38–2 record, the 2013 NACA Division I Tournament title, and the 2013 National High School Invitational semi-finals. He was named to the NACA All-Tournament Team and earned NACA Tournament MVP honours.
Ferguson was a member of the USA Basketball Men’s U16 National Team that posted a 5–0 record. Winning gold at the 2013 FIBA Americas Under-16 Championship in Maldonado, Uruguay. Ferguson played in all five games and averaged 5.8 points, 3.8 rebounds and 2.0 assists per game.
The following year, he was a member of the USA U17 World Championship Team that posted a 7–0 record and captured the 2014 FIBA Under-17 World Championship gold medal in Dubai. Ferguson played in all seven games (with four starts) and averaged 9.0 points, 1.3 rebounds and 1.6 assists per game.
In July 2015, Ferguson won his third gold medal in as many years after helping Team USA go 7–0 at the 2015 FIBA Under-19 World Championship in Heraklion, Greece. He played in all seven games and averaged 6.1 points, 1.6 rebounds and 1.0 assists per game.
Heading into his senior year of high school, Ferguson was ranked No. 10 overall in the ESPN 100.He averaged 17.0 points, 3.5 rebounds and 2.0 assists per game for Advanced Prep in 2015–16, earning a trip to the McDonald’s All-American Game.