K-On! (けいおん! ,Keion!) is a Japanese four-panel comic strip manga written and illustrated by Kakifly. It's a slice of life story about girls who formed band in their school music club, which now has an anime adaption.
Ok now on to the ecchi part!!!
Mio's the most popular girl on K-on, she has the most fan art and other ecchi stuff like this one. The sone is "Don't Say Lazy" which is the ending theme song of K-On.
Opening Anime - Cagayake! Girls
Ending Anime - Don't Say Lazy