Arata Naru Sekai - Kako Hen

We came from the far future, where most of the adults have disappeared as if they vanished into thin air and all of the children are managed by a few adults. To discover the cause behind human extinction and to evade it from that root cause... That's what we had jumped to the past for, supposedly.
A boy sitting in the seats of a movie theater fidgets with his cellphone. "Before aptitude or anything, trying to change the future with just five kids? You would think there's a limit to lack of planning," he typed on his cellphone.
The boys wonder if they had really come to the past. Why? Because, ever since they arrived, they haven't stepped one foot outside this theater.