The obesity problem can be decreased by reducing the amount of fat foods the individuals consume; however it all still relates back to the motivation of the person.
People can have food addictions to my knowledge, just as they may become addicted to cigarettes or drugs. They may enjoy the taste of the sugar or carbonation they intake.
Schools may offer a nutritious meal, but that doesn't mean it is good quality. I would change it by increasing the quality, even if I paid extra. I see many kids who throw away their lunch because they think it's inedible.