I don't believe that. Lunch periods are typically 20 mins. My sons, when younger, could wolf down foot long subs in less than 5.
Sugar is an acquired taste and often products with high sugar content also contain a high fat content. Kids crave sugar and get the fat as an added "bonus". Then when the sugar high wears off, they feel ravenous again and stuff down more sugary food.
Look at cultures which limit intake of sugar (Chinese, e.g.). They are not overweight.
Adults are the same--want something sweet as a snack or after meals. If you cut out sugar for a few weeks, you cease to crave it.
I wouldn't know, as my children don't attend school.
Carbohydrates aren't converted to glucose? Asian diets are very high in carbs. Lot of rice and noodles. Sucrose (sugar) is converted to glucose as well. This is not so much about the quality of food as it is about the quantity consumed and a lack of exercise.