Lots of kids wear things like hemp necklaces and don't smoke pot. But- the school must feel fairly certain that your granddaughter is smoking or they would not accuse her. I understand you are upset with the school- and they should have more to go on than a necklace and a 'laid-back' attitude, but you need to ask yourself why ELSE they might suspect her?
If her friends gave her a 'stoner' nickname, that would be a warning sign to me. I know she is probably a good girl and trying not to disappoint you, but it is hard for kids not to slip up sometimes. I am sure if she was smoking again, she would go to any lengths to hide it from you because she doesn't want to disappoint you.
I think you need to take a step back from this emotionally and talk with the school again. find out if another student said your g-daughter was a stoner, or what other reasons they might have for suspecting her. Then- what I would do- take her to your family doctor and ask her to take a drug test PRIVATELY. Keep the school out of it for now.
If she has not been smoking, she has nothing to fear or hide. If she refuses to take the test, remind her that she may have to take one in the future to get a job, etc. and that you want to be able to clear her name at school. If she still refuses, get her some counseling, because that is a big red flag that she is hiding something from you.
Good luck- I know you are a loving and concerned grandma trying to defend her and do what's best.