The episode explores Kurumi's past, specifically her crush on an upperclassman (Senpai). During the initial outbreak on the school roof, he was bitten and turned into a zombie. Kurumi was forced to kill him with a nearby shovel to save her own life, an event that continues to haunt her dreams.
Watch a review of Episode 2 that explores Kurumi's painful backstory and the episode's themes: Gakkou Gurashi! Episode 2
Sharp-eyed viewers on Reddit noted subtle clues regarding the nature of Megumi Sakura (Megu-nee), such as other characters not directly interacting with her or her lack of a water glass during meals. The episode explores Kurumi's past, specifically her crush