who's there?

sara and her husband held a play rehearsal at school yesterday. they had got rid of the boys and the school was empty. they were in the corridor looking at past school photos when they both heard footsteps in the dining room behind them. the door handle rattled, as if someone was trying it. they both turned to see who was going to walk through the door - but no-one did!
they asked 'who's there'.
no response.
they opened the door to find an empty room!!
the school is known to have ghosts as several members of staff swear to have heard footsteps, a baby crying and seen apparitions.