According to the cover note of the seventh volume of the comic book, another OVA of Seitokai Yakuindomo
was announced. It will be bundled with the eighth volume to be released next year.
Seitokai Yakuindomo OVA official website
Source: Scan of the announcement
Update September 21
According to Weekly Shonen Jump, two more OVA will be released in the spring and summer of 2013.
Source: Scan of WSM
Update March 7th
According to the official website, the release dates of the two OVAs are May 8th and July 10th.
Sources: Seitokai Yakuindomo official website
, Amazon Japan