Are you factoring in your breaks and the times you stop to give direction? Have you done a run-through, in which you do the play from start to finish with no breaks? That will give you a good sense of how much time you'll need. How much time are you using between scenes and acts? Can you speed those up? How long is your intermission?
I taught 8th grade drama last year. We did A Christmas Carol, and in every rehearsal, the play took exactly one hour, day after day. On the day of the performance, however, it took only 20 minutes--and nothing was left out!