Grad Nites have been happening at Disneyland since 1961. Which makes 2016 the 55th graduating class enjoying Grad Nites at Disneyland.
I have a neighbor who works in TDA in upper-middle management but started as a summer job in the park in the 1970's. At a barbecue a few years back she told me that if Disneyland had done random drug tests on the CM's back in the 1970's that Disneyland wouldn't have been able to open most weekends due to lack of staffing. I suspect the WDW CM's weren't/aren't that much different than the Disneyland CM's.
And even in the 2010's Disneyland and WDW still don't drug test hourly Cast Members.
Grad Nite kids smoking funny cigarettes without filter tips, or smuggling in minibottles of liquor, or getting to Third Base or beyond in the back row on Pirates of the Caribbean are the least of the worries. I think it's the Cast Members that are having the most fun!