The second season of Highschool DxD
was announced in the OVA
bundled with the 13th volume of the original novel.
Highschoold DxD official website
Source: Screenshot of the announcement
Update Sep 6
Acccording to Fujimishobo, a new OVA of Highschool DxD
was announced to be released in May 2013. It will be the unaired episode 14 of the first season and will be bundled with the 15th volume of the novel.
Source: Scan of the notice
Highschool DxD Dai 2-ki