In the case of a national emergency the weight and drug use stuff would be tossed aside. Most kids are in a physical state that could be cured with some basic training and diets.
The drug abuse issue is different—but the mil standards are much stricter than most. Where pot is legal the ingestion of it would be disqualifying. In reality, that shouldn’t be a huge issue.
The bigger problem would be during a draft, actually getting the kids to show up would be an issue.
With student loans being forgiven, I am not sure what the attraction would be for a lot of these kids. Of course the tech schools are fine, but kids don’t want to be involved in endless wars and deployments. And their parents haven’t instilled a culture of service. So...those things are much more critical.
This was a big problem during WWI, hence the formation of the American Protective League, and "Slacker Raids". You could literally be stopped anywhere, any time and be forced to show your Draft Registration Card.