Toaru Majutsu no Index SS

Volume 1 (Set between volumes 13 and 14)
A series of short stories set before the events of volume fourteen including how Team GROUP was formed.

Volume 2 (Set between volumes 16 and 17)
A series of short stories including two separate meetings between Touma's and Mikoto's parents, two tales involving two of the Level 5s of Academy City, and other funny stories involving some of the characters.

(Source: Wikipedia)

Toaru Majutsu no Index SS was published in English as A Certain Magical Index SS by Yen Press under the Yen On imprint from November 17, 2020 to March 2, 2021.