On November 5, 2018, after skipping his junior year and a part of his senior year to play professionally, Ball joined SPIRE Institute, a prep school in Geneva, Ohio, where he played under head coach Jermaine Jackson. SPIRE competed outside the jurisdiction of the Ohio High School Athletic Association, allowing Ball to play without concerns over his amateur status. A number of prominent high school teams canceled their match-ups with SPIRE because Ball's professional experience would threaten their eligibility under their state federations. His team was removed from the Hoophall Classic tournament because event organizers did not meet a $10,000 request from a Ball family associate in order for Ball to play. On November 10, 2018, Ball debuted for SPIRE, recording 20 points, 13 assists and five rebounds in a 96–84 win over The Hill School. On March 7, 2019, he scored 41 points, including 29 in the first half, in a 102–67 victory over Hillcrest Prep North at the Grind Session World Championship. Ball helped his team reach the final and was named tournament MVP. He was ruled ineligible for the 2019 McDonald's All-American Game due to his professional experience.