User blog comment:B214/Light Novel Volume 16 spoilers/@comment-23847729-20140128181036/@comment-1359322-20140131182109

The third semester is only a short one. Plus not all schools run a three semester system.

Assuming if there are a three semester system.


 * First term from April 1 to late July, then Summer Break
 * Second term lasts from early September to late December with a two-week long break for New Year's at the end of the year.
 * Third term from early January to late March and a brief week-long spring break. The graduation ceremony occurs in March, and the enrolment ceremony in early April.

Basically the third semester isn't really a study period, it's more like a period where 3rd year students passing the jobs and all to 2nd year students. In that sense, Rias and Akeno would probably be graduating in Volume 20. Issei and the others would be 3rd year student in Volume 21.