Assassination Classroom manga
I was wondering after you've finished the series where do I start reading in the manga? Or should I read it from the beginning?
I would recommend always reading the manga from the beginning since most anime don't always capture all the events that take place in a manga. But seeing as I don't read the Assassination Classroom manga I couldn't inform you as to what chapter the anime is currently at, but perhaps another user might be able to help you out with that.
If you want the licensed english translation, Volume 4 is the most recent one and that is only at Episode 12 Ballgame Tournament Time.