Corporations claim this is a freedom of speech issue. Certainly some examples are more egregious than others, but essentially, having ads in schools lets outsiders decide what goes on in the classroom, and commercials in the classroom blur the line between propaganda and education. And, of course, parents and the kids pay the price. Many parents are just wringing their hands over their kids' extreme consumerism, because commercialism creates enormous appetites in the young. They've got to have the stuff to keep up.
Commercialism reduces the human quest for transcendence into a desire for more stuff.
It hypnotizes kids, making them crave stuff that will never satisfy - setting the stage, I would argue, for drugs and violence.