Question: What traumatizing experience did Reid face in high school?
Answer: The football team publicly tormented him
In Season 3, Reid opened up to Derek Morgan about a very painful high school experience. Reid was told that the most popular girl in school wanted to meet him behind the field house, but when he showed up, the entire football team stripped him naked and tied him to a goal post on the soccer field. After hours of torment, Reid finally freed himself and walked home.
"Elephant's Memory" (Season 3, Episode 16)