Glad to hear you're enjoying the game! To unlock that CG, go to Matt's place in the evening to hang with him & Leo. Midway through when you're asked to take your shirt off, go back to your dorm instead and that CG will unlock.
Thanks so much! Can you spoiler us who the 9 romanceable characters are? I read in the other comments, Griffith and Riley for sure. Marco, Leo, Evan, and Asher are polite valid guesses I suppose. Who else? :)