I'm pretty sure the guy in the photo is the studying kid who used a fake name to escape pressure from his dad, and Taehoon wanted the photos because he's the mega gay
@DoubleK - jokes aside, Taehoon is in the photos, too, so Yeon Woo would know him if he were that kid.
But given that Bomi probably mistook Hobin for Dowoon, my guess is that she, Taehoon, Dowoon and Yeonwoo went to the same middle school, even if they didn't all know each other.
This chapter is full of golden lines, between 'Where are my knuckes ?' 'Is this a ping-pong ball ? It fit in one hand' and 'Mom, that dude beat up his classmates', I was dead the whole chapter.