Just went back and replayed my save from before imprisonment.
New summary of sequence:
Casino and below ground sex dungeon are extension of Vice Principal storyline. Once you have his other 4 scenes, click Heart at first floor school restroom for new scene. (If going for NTR and rescue attempt, wait until NTR points are over 100.)
A Heart appears next to bucket in your school prep room. Clicking that gives scene where VP screws her from behind while she looks out window at students leaving school.
Then he will ambush you after class one day and insist you meet him at casino that night. When you go to bed, you will awake and go there for a scene.
Unsure of trigger, may be repeating restroom and window scenes or rise in lewd level,
but VP will appear near stairs on second floor of school with a heart icon over him. Click him for gangbang scene in classroom which ends up with scenes in basement and imprisonment there.
My lewd level went from below 300 to above 400 in process of getting the VP appearance in hallway to trigger.
Updated my
Routes Walkthrough with this and some other info.
