User blog comment:B214/Light Novel Volume 16 spoilers/@comment-23847729-20140128181036/@comment-1359322-20140201014241

Jop, like i said, most of the 2nd year students are already considered as third year student in the third semester, it's like a probation period for them. The 3rd semester is also a period where the senior leaves their job/position to the junior. For example, in the third semester, Sona and Rias needs to appoint a new Student Council President and Club President.