Wisconsin recruited Platter as an outside linebacker out of Menomonie High School in northwestern Wisconsin. That school has been good to the Badgers in recent years, as Platter is now the third Menomonie native on the roster, joining inside linebacker Mason Stokke and offensive lineman Alex Fenton.
He was a dangerous pass-rusher and playmaker throughout his high school career, tallying 27.0 sacks and 23 forced fumbles. Last season as a senior, he was named first team all-state by the Wisconsin Football Coaches Association and was a finalist for the John Anderson Award, an honor given to the top senior linebacker in Wisconsin.