I think if you're going to say smoking weed is a gateway to other drugs, then you should ban smoking cigarettes and alcohol too, because it's all on this metaphorical ladder.
I've smoked weed before, and at no point in my life would I ever consider trying anything harder than that. eg coke, except maybe psychedelics, because I'm aware of the health effects of weed compared to other drugs. But I'm not everyone, so you're right that there would need to be education about such things. But there's already plenty of education about the effects of cigarettes and alcohol, so I think this would just take a similair path.
You're right that the culture and usage of weed is bad, but I think that's because of its criminality associated with it. I'd actually legalise weed for a whole host of reasons, because it makes it less dangerous to get, takes business away from criminals, you can regulate it, it is factored into our economy, etc.
But the weed point was really just me rambling and thinking of things to say, ultimately to prevent knife crime the biggest thing is we need increases in police funding.