At the age of 14, Simmons was torn between his two sporting loves of basketball and Australian rules football, but eventually decided to focus on basketball. Simmons later revealed it was his football coach's decision to play him in the position of ruckman that swayed his decision to pursue basketball. The following year in grade 9, at the age of 15, he played basketball for Box Hill Senior Secondary College at the 2011 Australian Schools Championships before taking up a scholarship at the Australian Institute of Sport in 2012. Later that year, the American dual-citizen Simmons made his first appearance in the United States at the renowned Pangos All-American Camp for basketball prospects. Despite still being 15 years of age, he was also chosen to represent Australia at the FIBA Under-17 World Championship and helped his team win the silver medal. Simmons' tournament highlights included a 26-point, 10-rebound, 5-steal outing against the Czech Republic.