Dowman Sayman manga
From Sugar Oasis: Nobushina Naharu has always wanted to become a famous detective. Her dream is to set up a "Detective club" at school, but the teacher requested that a club has to have at least four or more club members to be considered a club. So then, when she has finally gathered 3 other partners, she realizes that they are getting weirder and weirder one by one.
Saikawa Rai was known as the Goddess of Luck throughout her school and nobody knew what the source of her luck was. Hanabusa Mitsuru confesses his love to her and finds out the source of her luck... Dice.
From MangaHelpers: 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! [tethysdust]