Now that our culture is finally confronting its irrational prohibition of cannabis, there remains the issue of "the kids". I've been a user for 50 years and I am quite certain that cannabis use is bad for young developing minds. There are probably cases where it would be very useful under the supervision of a professional, but not recreationally. It distorts the lessons a young mind must learn, it confuses rewards systems, etc. It's not good. Period. So, when we're talking to concerned parents, there's no point in extolling its virtues or trying to ignore its harms. It's so easy for those of us who are adult users to try to ease their fears, but their fears are real. Many a good child has lost his or her way using cannabis.
Instead, make them focus on the way things work under prohibition. For decades, marijuana has been available in the black market. We all know this, right? All moms know this. And all moms know that their kids can buy it in school, presumably from "bad kids". If you pin them down, they know that either their own child or a friend can approach one of these bad kids and make a deal. They also know where this pot comes from. It comes from criminals - older kids - and those kids get it from ... gangs. Right? Yes. They know this.
So the choice, the real choice, is whether parents want their kids buying weed from bad kids who are associated with criminal gangs, or some neighbor whose surplus might make its way to their kid. The local grower isn't involved with gangs and the other enterprises they indulge in, like prostitution and hard drugs, and none of the money he might earn by skirting the law goes to finance those activities. In fact, it's likely to be spent locally on groceries and rent.
Ask them what kid they think can't buy it now. Make them really ponder it. Make them describe this kid that can't buy pot. That's the stumbling block. They prefer to think that it's hard for their child to buy it now. But seriously, how is that possible? So yeah, maybe their kid could get it any day he wanted to. Make them own that knowledge.
Then ask them where they want it coming from, and what kind of person they want their kid interacting with. They're going to buy it if they want to. Do parents want it coming from a dispensary or a gang? Do they want it to be untested and unknown? Should it come from some friend's older brother who bought it legitimately, or ... from a bad kid with gang connections?
Middle school kids have some of the best connections in town. The idea that it would be easier for kids to get if it was legal for adults, isn't rational. It amounts to regulation. What parent wants it to remain unregulated? Keeping it illegal forces their kids to interact with the very people parents try to keep their children away from.
It's a no-brainer when you present it to them that way. It may take awhile to sink in, but they'll eventually get it.