Ooh, I think I know which scene you're talking about--the one where Onozuka and Iwaki conspire to teach Miyasaka a lesson, right? That was great! There's also the scene towards the end of the book where Iwaki initiates sex in the car after the meeting with Mochimune. Yep, his exhibitionist side is definitely coming out! ^_^
I'm just starting Book 12, so I'm looking forward to reading more about Mochimune.