Akane and Megumi are thinking about this strategically – they realize that their chances of accidentally walking in on two guys making out increase exponentially if the guys can’t hear them approaching. You can’t leave accidents to chance, you know.
And speaking of chance, the two know that they have no chance of making it through high-school without accidentally walking in on something. It’s just too much of a staple of high-school anime/manga to spare anybody in the mid- to long-term. So, if they don’t make sure that they’ll walk in on something they’re actually interested in seeing soon, the chances increase they’ll end up walking in on something that isn’t up their alley. So it’s really not lewd or pervy, it’s just taking care of themselves.
And, yeah…Kunoichi were pretty darn rare in real life, but in any comedic or comedy-adjacent high-school manga or anime, all girls can be safely assumed to have on of those pink ninja outfits in their closets, just in case they should ever need them. That’s the reason why nobody is ever surprised at their appearance.
As for Mopey, her impression in panel two seems to indicate quite clearly that she feels a bit guilty, or at least conflicted, about the manipulative way she steered Youichi and Hiroki toward their current circumstances… …but really it’s just a bit of acid reflux, she ate something that didn’t agree with her.
More on Monday.