I need help finding a horror manga I've read a long time ago but no matter how hard I tried, I couldn't remember the tittle. It's about a girl who moved into a girl dorm and girls only high school in the middle of a big forest. The dorm/school was a very old creepy mansion and at night the surrounding forest was pitch black. I remember the main girl have nightmares about the forest. All the characters dress like in the old days in english and the drawing style was really creepy. If anyone know the name of this manga please let me know.

I'm not sure how long you mean by "long time ago", but when you say creepy mansion, old fashioned dresses, etc., I immediately think of Dolls Fall.

Its an OEL rather than a manga, but it might be The Dreaming

I just remembered another manga that you might be thinking of: Bradherley no Basha.