Nikutai Ten'i

Ken'ichi and several of his friends gather at their school during summer vacation to look at a new archaeological find, a bizarre looking mirror. Suddenly, the entire building is transported to an alternate dimension and a magic field surrounds it to prevent them from escaping. Also, their minds have switched to other people's bodies. They now must find a way to return everything back to normal until the dimension falls apart.

[Edited from ANN]