The Red Night — it appears without warning. Real, yet unreal. A dark world which exists in an alternate dimension. The Fragments, boys and girls who have been dragged inside are being attacked by strange black knights. Why them? Just what will happen? Will an answer await them at the end of this nightmare? — from intro

Seven teen classmates are transported to another world magic where they must defeat six black knights in order to return home.

[Source: AniDB]