heres what can happen if you arent honest:

my ex was in alcohol counseling. this kid named mike was in there but also for narcotics and alcohol. they did random drug testing at the counseling sessions and he had relapsed and smoked pot a few days before that. well, instead of doing the normal thing and getting someone elses urine he took water and put a tea bag in it to make it look yellow lol. they used pH strips that are meant for human urine. so when they stuck it in tea it showed up that he was on narcotics, opium, lsd... all kinds of stuff! he explained it to his sponsor and he was fine.

moral of the story, the lie is always worse than the crime. any understanding person is going to understand a smoke up on a birthday.

good luck. (sorry, i just forgot about that story til now)