We just got back from an 8-day trip that included both weekends of Grad Nights.
Magic Kingdom officially closes to the general public at 7:00 pm.
However, they do not begin letting in the grads until 10:00 pm. This gives them a chance to clean up, re-stock, etc. The party for the grads is officially from 10:00 pm until 4:00 am.
We stayed at the Poly, and had a letter from the management that warned us that fireworks would be going off at midnight on Saturday night (of both weekends). I'm glad for the warning, as it was loud enough to wake us up. And it isn't Wishes; it is just a fireworks show.
I was pleased that other than the fireworks, we didn't hear anything else; (we had been told it could get loud - even at the Poly).