Tenshi no 3P is not only just Ro-Kyu-Bu in a Jpop-rock band, they even featured the anime in one of the episodes!
Both shows you get enthusiastic lolis, and both of Kanae Itou's characters here are major-supporting characters who secretly loves the [boring] protagonist.
Both are about a somewhat bland high-school protagonist coaching little girls in music and shogi, respectively.
If cute lolis who are insanely overpowered in their trade is your thing, enjoy!
Beware of sexualised children (no surprise though, this is anime after all).
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.
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.