In his freshman season in 2014–15, Clark played basketball for the Buckley School, averaging 15.2 points and six assists per game. He helped his team reach the CIF Southern Section Division 4A semifinals, scoring 16 points in a 69–51 loss to Campbell Hall School, and collected All-Liberty League honors. For his sophomore year, Clark transferred to William Howard Taft Charter High School in Woodland Hills. In the 2015–16 season, he averaged 12.1 points and 6.9 assists, earning team most valuable player (MVP) and all-conference recognition. As a junior, Clark averaged 18.9 points, 7.1 assists, and 4.2 rebounds per game, winning team MVP, all-city, and all-conference accolades. During the season, he surpassed the school record by recording 22 assists in a single game. In the summer of 2017, Clark was named MVP of the Nike Elite Youth Basketball League (EYBL) Peach Jam after leading his Oakland Soldiers team to a title. In his 2017–18 senior season, he averaged 19.4 points, 7.2 assists and 2.9 steals, shooting 41 percent on three-pointers. Clark was a consensus three-star recruit. He first verbally committed to play college basketball for UC Davis but decommitted in August 2017 because he felt that he could play at a higher level. On October 2, 2017, he committed to Virginia after also considering Gonzaga and UCLA.