Mirai wa Bokura no Te no Naka ni

In girls school there is a legend that on a full moon night there are 13 stairs in a staircase of an old school building instead of 12, and that the curse of the city is linked with these 13 stairs.
On a full moon night two weeks before cultural festival main character witnesses a girl staying on 13 stairs. Has she got cursed? As school festival draws near, mysterious phenomena occur one by one. Will protagonist be able to lift the curse from the girl with the help of his friends before the festival?

[From vndbreview]