Kajitsu

Main character's parents divorce, so he has to move to a rural school. After the initial week he gets disenchanted with the school and his classmates. One day he notices a shadow in the school building. Protagonist decides to investigate it and finds a girl sitting alone in a classroom dyed in the red sunset. She does not notice protagonist and leaves the classroom. The next morning he learns that girl is a local mentally ill weirdo Chihaya who is of school age, but does not attend school. He starts to get interested in this girl.

[From vndbreview]