User blog comment:B214/Light Novel Volume 16 spoilers/@comment-23847729-20140128181036/@comment-16281343-20140131183241

Oh. So, the preview that they released on Volume 17 is wrong?

Also Volume 17 would be he end of the second semester and is Rossweisse's volume. Volume 18 would be winter holiday and is Irina's volume. Volume 19 would be the start of the third semester and is Xenovia's volume.

~ I know how their school system works.. no need to be too technical..

No matter what they do in their third semester.. Issei and the others are still 2nd year students until the new academic year arrives.