I can clearly see the popular opinion (reddit/hacker news) is that marijuana is harmless. My professional opinion is different, however, and over the course of my career I have seen its affects first hand. Laugh and down vote me if you will, but the negative affects are real and damaging to youth and the research backs me up.
My teenage relative's schoolmate went from being an excellent student and actress to a severe down-and-out in a short space of time due to her marijuana use. Likely other things contributed but the change was radical.
Without administering chemical tests to all of your students, you can't know with certainty which ones are using.
Sure, if a student has problems from it you will be able to observe that, but it's flawed reasoning to go from "I observe some students having problems" to "Using always leads to those problems".
And I don't naively believe that marijuana is harmless, but if you give me a choice between prohibition for everyone and (further support for) programs that reach out to students that exhibit sudden changes in behavior, it isn't very hard to decide.
Heck, these kids shouldn't be drinking coffee or caffeinated soda, either.