Kimi no Katachi ga Utsuru

Main character is a ghost member of science club. One day he is called by his sempai to investigate mysterious scientific phenomenon at school. He goes with two friends at night school and witnesses some male and female students have sex there. On the next morning it turns out that that at night there was a phenomenon of mirrors. There are various rumors about doppelgangers, death, happiness and other dimensions linked to the mirrors. The investigation is only beginning.

[From vndbreview]