Has Azumanga Hoshuu Hen been released in English?
As of this answer, Yen Press has not released the redone release of the series in either the UK or the US. They did however release the Omnibus, but that was before the 10th Anniversary of the series (and release of the manga, also why Yotsuba&amp; went on hiatus). 

The entire series was re-released as 3 volumes (one for each year), The author redrew a couple more chapters (16 pages of new content in each volume) to go at the start/end of the new volumes (these are the Supplemental lessons).



The author also redrew a lot of the content. Sakaki and a lot of her fanservice shots are redone. Some of the punchlines were also changed or completely redone  (e.g. Kimura doesn't appear at the end of the Christmas chapter).