The Locked Floor

Karasuno High has theee floors.

Everybody knows that. Friends, Family and even outsiders. It's clearly seen from the outside.

It's on the school maps, websites - everywhere.

One afternoon, as the four first years were cleaning up after club activities, they go back to class as Hinata forgets his stuff.

Being tired, and Hinata forcing the other three to come with him, they walk upstairs.

One of them notices something.

The staircase sign says :

2 -> 3 -> 4

They brush it off, thinking of it as a prank or maybe they're just too tired.

After they retrieve his things from the third floor and as they head to go back down.

They see a staircase which was never supposed to be there.

Sure it might be going to the rooftop, but it was clearly seen that the rooftop door was not seen and just another hallway.

What on earth was happening?