Anyone in their class knew about Katsuki's and Izuku's rivalry, and the fact they were frenemies.

A quirk that could reveal Soulmates was randomly leashed out upon the whole Japan, and everywhere, the same color of a strand of two people's hair proving it.

And guess what.

Izuku and Katsuki had the same color of a strand in their hair; a pale pink strand, unable to hide. They were soulmates.