Can't believe nobody's mentioned 'The Girl from Random Chatting' yet. It's so much more than the generic title suggests. Yeah, the premise sounds like a silly romance app story, but it gets surprisingly deep into the realities of online bullying, social anxiety, and the pressure to maintain a facade in high school. The main character, Junwoo, is painfully relatable—awkward, trying to fit in, and making a ton of mistakes. It deals with heavy themes in a way that feels grounded for a teen reader, without being gratuitous.
For a lighter, more adventure-focused pick, 'Omniscient Reader's Viewpoint' is fantastic, but the webnovel is the superior experience; the manhwa is a solid gateway. 'Eleceed' is pure comfort food—superpowered students and adorable cats. It's hilarious and the action sequences are clean and easy to follow. The friendship between the main guys is the heart of it. Avoid the super violent or overly fanservice-y stuff that's creeping into some mainstream titles; teens can spot hollow shock value a mile off.
Honestly, my younger cousin got into manhwa through 'Unholy Blood'—it's a vampire revenge story with a female lead who kicks serious butt, and the art is cinematic. It wraps up neatly in one season, which is a blessing.