How can I get into the auditorium?

Hint 1 of 7:

There's currently no concert going on there, so there's no reason that you
should go in there yet.

Hint 2 of 7:

But if there were a concert, you probably could go in.

Hint 3 of 7:

Notice that the poster for Alexandria's concert is pretty worn out.

Hint 4 of 7:

Why not try replacing it with a new one?

Hint 5 of 7:

Look on Alexandria's bed, which is on the far end of the room on the second
floor that's full of beds.  You'll find another copy of the poster in a
portfolio.

Hint 6 of 7:

Once you put up the poster, the ticket booth opposite the study on the first
floor will open.