Alcohol and cigarettes are already regulated because of health concerns and problems
God, wouldn’t it be awkward if just a few decades ago smoking was actively promoted as being healthy? As for your gem about “food is exempt”, that makes no sense; practically everything organic can be “food”. Alcoholic drinks are literally food, they have a caloric content and usually includes some sugars and minerals, how is it not food? Hell, puffer fish is also a food, you think schools should be serving that?
How is this so contentious? Excessive sugar is known beyond a doubt to be unhealthy; how is it controversial that schools the place you trust your kids will be safe, get educations, stay healthy, would try to limit sugar intake?
You’re right, idiot and neglectful parents that don’t care about their kids’ well being can keep on ruining them out of school grounds, but while the kids are the responsibility of the school, we should hope that they act responsibly.