Humber attended Carthage High School. The Carthage Bulldogs had won the state title in 1990 and been one of the final four teams in the state tournament in 1994. Humber was promoted from junior varsity midway through his sophomore season. He compiled a 4–0 win–loss record with a low 1.09 earned run average (ERA). He went 7–2 as a junior, and posted a 13–1 record with a 0.90 ERA and 146 strikeouts as a senior. A capable batter, he also had a .371 batting average with six home runs. Humber threw a shutout in the Texas University Interscholastic League Class 4A semifinals against Cedar Park High School in 2001, his senior season. Although Carthage lost the state championship to Western Hills High School in baseball that year, Humber was named Texas High School Class 4A Player of the Year by the Texas Sports Writers Association. According to former Major League Baseball prospect and East Texas native Alan Move: "Back in high school, he (Humber) threw harder than everyone else and changed speeds pretty well and that's all you need in high school."