Not all schools, just schools that actually need it. Some kids know better. They may have good examples at home while some don't. Obesity might also not be as rampant in their school district. My district is cautious and aware of it but we don't seem to have that problem if we compare class pictures to let's say.. From a district somewhere in Mississippi.
Junk foods are calorie dense but lack micronutrients. Tons of carbs and sugars which our metabolism processes right away just as fast as it stores whatever that is left over around your waist and thighs. Chips, soda, and candy. Things that when you look at, can't tell its caloric value by proportions. "Its just small bag of chips" omnomnomnom. But it's not going to make you as full compared to let's say... An avocado.