Tenshi no 3P!

Cute little girls being coached (or helped) by a high school guy. Moe overdose warning. If you liked the lolis of one of these animes you will also like the lolis of the other anime. 

These two series are not only almost similar in their premise and characters, they are also adapted by the same studio, therefore, those who enjoyed the first show, definitely will like the second. 

Cute girls play rock and create music. 

both focus on cute girls with various specific characteristics playing in a band 

Not sure why no one made a similar recommendation yet. MCs are socially isolated people,escaping from reality, not attending school in fear of being judged by peers. Both also have a good amount of bullying. Sakura and Anjou are pretty much the same person just really really similar and they both want MC to come back to school. 

Bunch of cute girls decides to start light music band. Lots of cuteness and moe. Main difference is that Tenshi no 3P! actually has a male lead, thus is a harem, while K-ON! is clasicall "Cute Girls Doing Cute Things" with only girls. Anyway, if you enjoyed one, you will most likely enjoy another.