Board Thread:Questions and Answers/@comment-30882723-20161231013441/@comment-5275759-20161231114014

Season 2 concludes with part of SS1 (after Volume 13). Remaining content prior to Shinyaku Toaru Majutsu no Index includes part of SS1, Volumes 14-22, SS2 and several other side-stories. The level of content is too much for a conventional two-cour season to cover satisfactorily (plus the story structure at that point is slightly problematic as well).