My Friend's Little Sister Has It In for Me! Volume 8 by mikawaghost

The biggest event of your high school career has to be the field trip! Akiteru and his classmates are off to Kyoto to deepen their understanding of Japanese culture. But with Iroha out of the way, Mashiro’s more concerned about deepening her (fake) relationship with Akiteru!

Too bad her plans are thrown into disarray when it turns out there’s a third girl vying for his affection. While Mashiro tries to come to terms with this new development, Iroha goes on a long-distance offensive to annoy Akiteru—but will that be enough to satisfy her?

With trouble like this brewing, the class’s “relaxing” trip to the hot springs will be anything but!