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.