Elena's best friend, Robbie, and his family move when the kids are ten. The separation is devastating to them both, and all the more so when abruptly Robbie stops communicating with Elena a few years after he moves back to Korea. It comes out that the reason for the silence is that while training for a K-Pop debut, he wasn't allowed to have a personal cell phone. No one told Elena that, so she felt abandoned, and never bounced back socially from being deserted. 

Then Robbie shows up out of the blue, showily asking Elena to make good on their childhood promise to go to prom together. Elena's shocked response isn't what Robbie expects, and awkwardness persists...and persists. I found the will-they-or-won't-they-but-obviously-they-eventually-will a little tedious. Still, I love a K YA, and this one is just fine. 

In addition to the romance, Once Upon a K-Prom has a "save the community center" plot that is endearing.