Ekudorado ~Kagami no Naka no Oukoku~

A young high-school student named Seirou has a normal, boring life: days resemble each other, and so do the nights: every night he sees the same strange dream: two girls talking to each other, one of them saying something about a coming darkness... One day, he and his female classmates find a mirror in an abandoned house; when they touch the mirror, they are transported into the magical medieval land of Ekudorado. The hero must first find his classmates, who are scattered around this world, and then find a way to return to our reality...

[From Moby Games]