Armstrong was a two-time All-League selection in both water polo and baseball at St. Joseph High School. He threw a no-hitter and drove in the game-winning run in a 1-0 victory over eventual-league champion Arroyo Grande. Armstrong also hit a walk-off, three-run home run to help the Knights beat Atascadero 5-4 to open the PAC 8 League Tournament. Walker hit .272 with two home runs and 19 RBI his senior season. He also went 3-5 with a 2.98 ERA and 33 strikeouts. He plans to major in computer science.