According to Natalie.mu, a new OVA of Sayonara Zetsubou Sensei
was announced to be released. It will be bundled with the manga volume 19. Since Zetsubou Sensei manga has been published every three months, it's scheduled to be released around November 17th.
The limited edition of the manga volume 18 has an extra DVD, which includes 13 OP & ED songs of the series and a preview of the new OVA.
According to the official website, the title is "Zan Sayonara Zetsubou Sensei Bangaichi" It's a two-volume OVA and the first volume will be released on November 17th.
Source: official website