Last year, I was invited to a party with six of my buds to go to Halloween Horror Nights right after school. We got to the park at 6 (when HHN starts) and stayed til 1 (when it closes typically)...then I waited outside my house for my parents to wake up and let me in the house
Anyway, we missed two houses, and one of them was a major attraction! The only stops we had were to ride Men In Black, the Mummy, buy T-shirts, and get ice cream, and since lines were short for everything BUT the haunted houses, that barely subtracted any time!
This was on a Friday night, but week-nights might be less busy...however, if you've got to leave early for school the next morning or whatever, you won't be able to stay long anyway. I would say GET THE PASS!!! You won't need an express pass for the rides, but I'd do anything to get to the front of the line on every haunted house and do all of them. Not only does it let you experience everything, but you can stop along the way and take time to enjoy the special shows and Scarezones.
After last year, I think you'd be dumb not to buy the express pass at HHN if you can