Legality of the drug doesn't matter if your employer or school considers it a prohibited substance. It doesn't matter if you smoked it in WA, in CO, in Amsterdam, International waters, on the moon, with a card, with an Rx, in a house, with a mouse, in a box, with a fox, etc.
If you flunk a drug test, the Supreme Court says you can be terminated on the spot.
However, WA and CO may become interesting test cases for things like hair testing. If your pee or blood test is clean, will it still be legal to fire you for using a legal substance if you are not coming to work under the influence? If your contract is silent on whether or not the drug is permitted if used in a legal fashion while NEVER testing positive at work/school, it could be legal, but I don't know case law. It will probably be similar to alcohol where you can use it, but you better not have a drop of it in any pee or urine test while on the job.
But, all that being said, employers in health care don't like hiring drug addicts. They are unreliable and a liability.
If a school did ask, it might be about illegal drug use or abuse of legal drugs. In which case, you are clear on one, but possibly not on the other. Once you get to student rotations, plan on drug tests. Not saying it will happen, but if you plan for it, you won't be surprised when it does.