McAllister starred at Whitefish High School, where he excelled as both a running back and a defensive back. However, it was on defense where he truly shined. Known for his bone-crushing hits and instinctive playmaking, he recorded over 100 tackles, 8 interceptions, and 4 forced fumbles during his senior year, leading the Bulldogs to their first state championship in over a decade. His performance earned him All-State honors and a reputation as one of the hardest-hitting players in Montana.