Roby attended Dixon High School in Dixon, Illinois. He joined a team that finished its previous season with a 1–28 record, but in his final three years at Dixon, he led the team to a combined 76–13 record. In his junior season, Roby averaged 12.5 points, 9.8 rebounds, 2.6 steals and 3.7 blocks per game, earning second-team all-state Class 3A honors. As a senior, he averaged 19.7 points, 10.5 rebounds, 5.5 assists and 3.9 blocks per game. Roby finished eighth in Illinois Mr. Basketball voting and was named second-team all-state by the Chicago Sun-Times. He left Dixon with the most career rebounds and blocks in school history and the second-most career points. Roby was rated a three-star recruit by ESPN and 247Sports, and a four-star recruit by Rivals. On October 22, 2014, he committed to Nebraska.
