i guess that would explain it lol, i was trying to trigger it for 3/4 ingame days guess i'll give up on seeing those scenes, i'm no1 to replay so much for just a couple of scenes.Check in the Recollection Room for hints (use the book in Serena's bedroom or the empty classroom at school). I don't think there's more to that one than just talking to them on different days while sufficiently lewd (revealing suit, no underwear).
May not be a repeatable event. I can't remember the chorus party stuff too well, though.