K-On Graduation
The K-On manga has been running in Japan for close to three years now, which as you may know is how long high school lasts in Japan. Well the series seems to be reflecting this in real time, as a recent chapter from the series has the girls discussing about going to university together with graduation in sight. The anime version adapted up to the point where the four main girls were already in their second year, so it only makes sense that by this point in the manga the girls are well into their third year.
You may remember that very similarly last year the Lucky Star girls were also graduating from high school. It seems to be a trend to take a story about moe high school girls & graduate them to higher learning. Which pretty much goes against the code of moe high school stories. Once you're out of high school you lose the school uniforms, which are quite important to the moe character. That & otaku are less likely to desire college-age girls (or older) as their waifu.
Source: http://www.sankakucomplex.com/2009/12/13/k-on-schoolgirls-no-more