Thompson averaged 16.2 points, 9.7 rebounds and 2.8 blocks while making 49 percent of his field goals for the 17-9 Rams of MICDS in St. Louis. Coach Matt Hixenbaugh's club was runners-up in the Missouri Class 4, District 6 Tournament. He also earned consensus all-state distinction.
He earned MBCA first-team all-state distinction as a junior in 2012-13 at Mary Institute/Country Day School after he averaged 13.4 points, 6.6 rebounds and 2.4 blocks per game. He was also a first-team All-Metro League pick and a St. Louis American Prep Athlete of the Week. As a sophomore, Thompson posted 8.0 points, 5.9 rebounds and 2.5 blocks per night while earning second-team all-conference honors and leading the Rams to a 15-9 record and conference title. In his first three high school seasons, he blocked 147 shots. He also plays for the AAU St. Louis Eagles.