Tldr:Aru borrow a gangster movie from Sensei to watch it with her friends. However, Sensei accidentally put a lesbian AV dvd he was gonna watch later in the DVD case Aru was carrying. Cue an awkward moment for the 68 girls watching the AV before they got horny and have some unseen Yuri action.

I think Mutsuki knew exactly what was inside that disk.