Magrisi, a freshman at Cal State Fullerton, attended Torrey Pines High School in San Diego, California. As part of a decorated high school career, Magrisi was named Cal Hi Sports Mr. Baseball State Player of the Year as a senior in 2018, after pitching to a 12–0 record and 0.75 ERA, while striking out 130 batters in 93 innings. In his high school career, Magrisi had a 28–3 record and 1.26 ERA, with 244 strikeouts in 217.2 innings pitched. He set a San Diego high school record in 2018 by tossing 54.2 consecutive scoreless innings, topping the previous mark set by 3-time MLB All-Star and 2-time World Series champion David Wells.