You have to do what's right for
you, not your parents. You are out of the house, self-supporting, in the workforce, in school - you are an adult. You don't need your parents' permission or approval for anything you do. If you spend your life trying to win your parents' approval, you will succeed only in making yourself miserable. So they're disappointed in you - so what? Are
you disappointed in you? That's what is important. It's your life, not theirs.
About the weed. I will take you at your word that it's not your weed. But your parents won't. Put yourself in their shoes: Would
you believe you? It's up to you whether to try to convince them.
Take care.