I don't think children are old enough to understand the long term impact of over-eating anyway - schools shouldn't really be selling food products which no absolutely no nutritional value (or items which individually are far more than a child would need to consume in one sitting) - massive king sized Mars bars I'm sure schools could do without.
It's a combination of things, poor education, poor choices, bad parents, poor quality food is cheaper, disingenuous food labelling, the lie that low-fat = good etc - none of these things in isolation if solved would achieve that much - but with a combined effort it may.
Sugar is a highly addictive substance, one which a scarily large portion of the developed world are addicted to - we wouldn't accept companies lacing food with cocaine to sell more products, so why do we accept the deliberate lacing of foods with salt & sugar to increase it's addictive qualities?.
http://en.wikipedia.org/wiki/Sugar_addiction
"Researchers say that sugar and the taste of sweet is said to stimulate the brain by activating beta endorphin receptor sites, the same chemicals activated in the brain by the ingestion of heroin and morphine."
"Finally, a 2008 study noted that sugar affects opioids and dopamine in the brain, and thus might be expected to have addictive potential. It referenced bingeing, withdrawal, craving and cross-sensitization, and gave each of them operational definitions in order to demonstrate behaviorally that sugar bingeing is a reinforcer. These behaviors were said to be related to neurochemical changes in the brain that also occur during addiction to drugs. Neural adaptations included changes in dopamine and opioid receptor binding, enkephalin mRNA expression and dopamine and acetylcholine release in the nucleus accumbens."