School physical and weed drug question?

I'm going for a school physical in 2 to 3 weeks and when they give me a cup to pee in can they check if I used drugs by the way if they can I only do like 4 hits of a blunt every month about 2 or 3 times since February and last week I did 5 hits of a blunt would 3 weeks be good enough so I can get clean I'm only 120 which is pretty light teenager and about 5'5 thank yous