"The Mirror will be moved to a new home tomorrow, Harry, and I ask you not to go looking for it again. If you ever do run across it, you will now be prepared. It does not do to dwell on dreams and forget to live, remember that."
-- Albus Dumbledore
Dumbledore explains what the Mirror does — that it shows the deepest desire of a person’s heart — and warns of the danger of losing oneself in the dream it presents. He informs Harry that the Mirror will no longer be found in that disused classroom.
So was the Philosopher's Stone already in the Mirror at this point? If not, where was it? --SVA