Pick and choose what you want to believe. If there were a story about how kids got better grades by eating pot brownies nobody could question that because it falls in line with the current legalization agenda.
Any study that says marijuana is bad is now some propaganda against legalization.
Strawman. This wasn't a "study", and as for your hypothetical story, I'd call B.S. on that too. But to try and reiterate the point - these stories were non-existent before decriminalization in Colorado, but now that it has been legalized, we start seeing the stereotypical, fictional nonsense that was foisted on the public in anti-weed propaganda videos in decades past.