Decker attended Rocori High School in Cold Spring, Minnesota. He was present in school when the Rocori High School shooting took place, hiding in a cupboard with other students until they were rescued by police officers. At school he was a three-sport athlete in football, basketball, and baseball. In each sport he was awarded all-conference, all-area and all-section honors. Additionally, he was named football team MVP two years in a row. During his prep football career he had 2,156 receiving yards and 28 touchdowns.