Arnold attended Kountze High School. On the basketball court, he was a two-time All-State selection, District 23-3A Most Valuable Player and named of the top 15 basketball players in Texas. During his freshman year, former NFL player Frank Middleton, who was an assistant coach, convinced him to join the football team. Arnold ended up becoming an All-State selection at cornerback, in addition to earning District Offensive MVP honors as a quarterback and wide receiver. He was a three-star prospect and committed to Baylor.
