Legal Firms Law Researcher Question:

If you are currently using any illegal drugs, then you will not pass the interview. You cannot break the law while at the same time seek a position which enforces the law. You should openly admit to any previous drug usage. Each agency has certain parameters as to the type of drug and the amount of usage they will accept. If you fall outside of these parameters, there is nothing you can do but apply with another agency.
If you smoked a joint a few times in high school or college, admit to it. This does not necessarily disqualify you. As long as the panel believes this was an infrequent occurrence in your past, and that you are now a responsible adult, you should pass this portion of the interview.