The argument is really over what "gateway" means. Pot does not cause people to become addicted to other substances, or even encourage them to try other substances. However, it is often a step on the path towards addiction, and therefore is a gateway to other drugs.
Someone who is looking for a high, whether as an escape or for fun, will generally use the most accessible tools, or the ones deemed most safe. In that regard, both alcohol and pot are "starter" or "gateway" drugs. That doesn't mean that drinking or smoking will lead to an OD, but that can be warning signs of a deeper issue. To that regard, there is great value in teaching teenagers about the dangers of alcohol and marijuana. To discount it by saying, "They aren't talking about heroin, so it's meaningless," is akin to not warning them about speeding because you haven't addressed drag racing or driving under the influence.
To say that pot use cannot lead to heroin is also a fallacy. How many people discovered they can mask their feelings and issues by drinking or smoking and therefore don't get the help they need? And since pot doesn't fix their issues, there is a chance that they will chase a greater high. Additionally, by hanging out with other people who may be doing the same thing, they now have a much greater exposure to the drug culture and harder drugs.
That being said, I agree with CV that I'd rather my kids smoke pot than drink alcohol. I have friends and family on all levels of the usage/addiction spectrum. Anything that affects your brain has an element of danger to it. Educating youth on responsible and appropriate use of those things is of paramount importance, and I'm glad the Chabad schools are starting to take it seriously.