If you include the fact that you never bought it, that's more information that affects the probabilities. It's just like in the Monty Hall problem where revealing a goat behind one door changes the probabilities of what's behind the other doors.
Given the fact that cannabis was recently made legal where you live, you may be 5% less like to pass a class. Given the additional fact that you chose not to use cannabis, you may be 5% more likely to pass a class due to the curve being lowered by those who do smoke.
I thought we already knew the academic impact of canibus use from the documentary Fast Times at Ridgemont High
I thought the characters drove their cars to school in that movie.