Is this a private school by any chance? If it is, something like drug testing is probably supported by the parents that pay for their kids tuition, so you would be better off changing their minds rather than school administrators. If it's not private, where do you live? What happens if you fail? They could never do this at my old high school because everybody would have failed haha

One thing you should include in your argument is that those drug tests most likely do not go over prescription drugs, and abuse of prescription drugs is becoming more and more popular - some say is the new marijuana, (click). It might be a good idea to at least bring this point up that prescription drug use is far more dangerous of an issue than smoking a plant.

Good luck, but you usually can never win fighting against your school. It's good you are staying clean yourself though, because regardless of what your own opinion is of using illegal drugs, a failed drug test would keep anybody important from paying any attention to what you have to say.