A footnote on the bottom of kakifly's K-ON! manga in the September issue of Houbunsha's Manga Time Kirara magazine lists the next installment as the final one. The September issue officially ships on Monday, and the next issue, cover-dated for October, will ship on September 9. The four-panel comedy manga follows a group of high school girls who form an afterschool rock band.
The ending of the current K-ON! manga series is not necessarily the ending of the K-ON! story as a whole. Tsukasa Hojo ended his Angel Heart manga series in the current (and final issue) of Coamix's Weekly Comic Bunch magazine, but he already announced that the story will continue.
Source: Uri-Sure Matome R via Anime News Network