Saikyou no Youhei Shoujo no Gakuen Seikatsu: Shoujo to Shoujo, Kaikou suru

Cierra, a young girl who's been a skilled mercenary her whole life, decides to retire from being a mercenary and try to live like a regular school girl. While at her new school, Cierra befriends Alna, a young noble girl who's being targeted by assassins, and vows to protect her.

(Source: MangaDex)