Classic case is short for "classic case of dogging the boys". To dog the boys means that one person of your friendship group ditches or dosent do something for or with that said frendship group for a reason less important than the group it's self.
Poulter: "Hey dude you wanna catch a movie with the boys?"
Mitch: "Na man I wanna catch up with my GF"
Poulter: ".....Classic Case..."