The issue here is one of correlation. You say "a lot of people are raised with tv and terrible schooling, and still manage to have the attention span longer than a commercial." Sure, and a lot of people smoke without getting lung cancer, but would you say that smoking doesn't cause lung cancer?Meesh mentions a problem in the school systems, and that children are becoming ADD because of television. I really don't believe that to be true. I think primarily it comes back to parenting. Responsible parents ensure a good variety in a child's experience, and teaches them what they need to know in order to make good decisions. A lot of people are raised with TV and terrible schooling, and still manage to have the attention span longer than a commercial. I would agree raising a thinking child would be easier with better schools and less mindless TV.
If you take a group of 100 smokers and 100 non-smokers many more smokers will die of lung cancer than non-smokers. If you take a group of 100 kids who watch a lot of tv and a group of 100 kids who don't watch much tv more of the kids who watch a lot will have attention problems.
There is a lot of psychological research out there to suggest that this is the case. One main problem with such research is lack of a control group. It's extremely hard to find participants who don't watch any tv at all!