Shinyaku Toaru Majutsu no Index: Birthday Through the Glass

Shinyaku Toaru Majutsu no Index: Birthday Through the Glass (新約 とある魔術のインデックス ガラスの向こうのバースデー) is a short story written by Kamachi Kazuma that is featured in Dengeki Bunko Magazine Vol. 61 alongside other stories from famous Dengeki Bunko authors.

Summary
Kihara Kagun, a teacher known as the Student Keeper who watches over the children of Academy City.

He was the strongest of his generation among the heretical Kihara researcher family and this is a story of him cracking down on the evil running rampant in Academy City.