This post seems more and more to feature K-ON!'s Yui and Azusa than any original characters... It's not from pixiv and no original tags can thus be associated with it, other than the obvious artist.

Now for evidence that this is in fact K-ON!... It's in the music room of a school... AFTER SCHOOL, as stated by the phrase houkago (Part of the name of the girls' band from K-ON). Metronomes and violas in the background only further suspicion. Last but least, the uniforms may resemble the girls' summer uniforms, but I don't really remember.

Tagging.