Grad night starts in MK after the park closes to others and ends before the park reopens the next day. I went many years ago, we were there just during that time, Grads only.
Most of the schools that participate are local and bus in the night of specifically for the event. You will probably only see a handfull of groups who are doing "senior trip" type visits. You have nothing to worry about.
I have been down several times during grad night and have never experienced any problems. The only issue is that MK closes earlier for the night for grad night to begin. Don't worry about it. You could always plan on heading over to Epcot after MK closes.
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