I was there on Friday the 10th (after 5pm) and Tuesday the 14th. The morning of the 14th was quiet till about 11am. After that I felt it became unpleasantly busy.
It's not just more people, but groups of students are loud and are very focussed on themselves and their group. Think: blocking streets, walking all next to each other so you cannot pass. I am sure they do not mean it, it happens at that age. However, it is a bit of a nuisance.
I would love to go back in coming years, but I will avoid Grad Nights, unless it cannot be avoided. A busy at Disney is better than no day at Disney.