AFO-Guard

Aizawa Katori goes to a school with an unusual rule. The nearby, separate school building is full of elite male students, and the school wants to prevent any of the female students from getting involved with them. For this reason, all the girl students are required to wear masks! Katori wears a horse-head mask, but she hasn't given up on getting through to her long-ago childhood friend and crush, Shinra. Shinra also has a crush on Katori from when they were children, but he does not recognize her as the strange horse-headed girl who speaks to him!

(Source: MangaHelpers)