I don't know how necessary drug screens are. I mean all camps do basic background checks and the like of course, but I think an element of trust would be lost between the director and the staff if an actual drug test were required. Just as long as it's clear that anybody who does use drugs during camp will be sent packing, I think the staff should be mature enough to handle that. (And if they're not- no loss to you)
The only exception to that rule was at a day camp I worked at, where the 21+ staff drove the buses that got the kids to camp everyday. Then by state law random drug tests were required like twice over the entire summer, but I can't think of anybody who failed which is as it should be.