Wagner attended Colony High School in Ontario, California, US. In 2008, he registered 125 tackles (92 solo, 33 assists), including four sacks as a senior. Also played tight end, tallying 37 catches for 595 yards with 11 touchdowns. He received all-CIF Central Division honors as well as all-Mount Baldy League accolades. Earned second-team California Division 2 all-state honors after helping lead the Titans to a CIF division title and was named to the all-San Bernardino County team.