Frazier, before matriculating to Oregon, attended Santiago High School in Corona, California. At Santiago, he was a 3-time 1st team All-League selection. Frazier was 7–4 with a 2.08 ERA as a senior, and was 8–4 with a 2.15 ERA as a junior. He was ranked by Perfect Game as the 25th-best pitcher in California in the class of 2018. Last summer, he pitched for the Orange County Riptide of the California Collegiate League, posting a 4.13 ERA in 28.1 innings of relief.