You will most definately be cleared out. I've been on both sides of Grad Night - back in my old high school days my school went there for Grad Night, and we were one of the first group let in. The park was empty for those first 100 or so to rush through the gates. Some of the rides are closed, they usually pump pop music loudly throughout the park, they put up extra decor for the theme (part of the reason you have to get out by 5pm, so they can be ready for that night), and once all the schools file in, it gets pretty crowded! I've also been up there more recently on a Grad Night, and indeed they close the park at 5pm (though it officially isn't emptied out for another hour or so, as usual). Everything shuts down and the cleaning crew start ushering everyone out with the big garbage vaccuums and such.
As for the special fireworks...a friend of mine was there for Grad Night last year (as an escort - she is a teacher) and she said the fireworks was done to modern music, and included a longer fireworks show...it wasn't tremendously different from the normal show, just longer, and the lights shined on the castle had 'Grad Night' and '2007' and such instead of just stars and moons and the like.