On the second thought, if this is presented with a straight face, then rivals look like targets at shooting range. What if like one of the girls is sick all the time and she comes back after illness, but still in a half healthy state, so she coughs all the time, walks to the nurse room and leaves a trail of napkins, which other students can use to track her down, so you have to take care of it too? Or one of them changed all the schools in the area, because in every school students hate her and bully the hell out of her, so it is hard to find her, cause she is hiding from everybody and she tells things like "I wish somebody like senpai loved me", which could make Yan-Chan feel for her. Or one of them is a fucking winner of a Royal Battle and if she sees you following her, she will go "oh, you wanna fight, bitch? It's on!!!" or that she can't be simply eliminated with a water bucket and shit. Damn, there is an entire ocean of possibilities to create diverse rivals with a personality and parody potential, but he decided to do "anime_cliche_name that goes to school_activity_name". Sorry, if I've become too autistic