A huge meteor could fall from the sky and land into the pool. Then, short, purple aliens come out from inside the meteor and basically say that they're there to come get EXO and bring them back to their home planet. Of course you and your friend freak out, but Chen and Baekhyun confess their love to each of you. Turns out, they're both Alien princes and they ask you two to go with them to their planet and be their alien princesses. You two reject because, duh, they're aliens. A spaceship appears in the sky, and Chen and Baekhyun disappointedly board the spaceship where they join the rest of EXO who is performing Call Me Baby. The song ends, they all pose coolly then the spaceship dramatically flies away.

No but seriously, an unexpected plot twist can make any story good. I can't really think of any from the top of my head, but I'll throw a couple things out there. Idk, you could turn the pool party into a murder mystery party type thing. If you make it suspenseful and scary that can create romace between the characters. Basically, it'll give you and your friend a chance to be lovey dovey with Chen and Baekhyun. Or you could just go totally cliche and create a love rectangle with Chen, Baekhyun, you and your friend.

I really can't think of anything good atm. I'm usually a lot better at these things. Sorry I'm not much help, but I hope I at least sparked your creativity. I'll check your story out once you post it. Good luck!