I second/third/fourth/whatever Shut Up: Flower Boy Band and Dream High (1, not so much 2 unless you REALLY love the first one). Also A Gentleman's Dignity.
As well, since most people seem to be recommending kdramas: Love Shuffle is a really good jdrama; it sounds like it's about relationships and couples and stuff, but actually, it's a lot about friendships, and the characters and their relationships with each other are just so quirky and loveable.
As for tomboy-ish female leads:
King2Hearts- Ha Ji Won plays a kickass North Korean military special forces operative; and it's a great drama
Wild Romance- This isn't for everyone, I'll be first to admit, but I found it funny and entertaining- don't let the title fool you though, it's not really that much about some crazy romance. It stars Lee Si Young as a tough, in your face bodyguard (she even looks like a boy in this drama).