User blog comment:B214/Light Novel Volume 16 spoilers/@comment-23847729-20140128181036/@comment-1359322-20140131184141

Not really, the third semester is like a period where students are being given time to prepare to enter their next year of study. So it's a in-between stage of the 2nd-3rd year for Issei and the others.

For First year and Second year student, it's like a preparation period to enter their next year. Well it's also a period to see if you pass or fail your test. If you fail badly, you'll have to remain for another year to repeat.

For third year students, the third semester is for them to prepare their graduation that's all. Same as the above, those that fare badly has to stay back a year.