Really, there is a study that says that. I haven't read the study; I have just read about it, which is not nearly as good, but what the heck, it's the day before Memorial Day weekend. Here is the story, so you can read it, too.

After noticing a statistical difference in test scores between the fast-food eaters and the slow-food eaters, Dr. Kerri Tobin of Vanderbilt University says, “It is possible that the types of food served at fast food restaurants cause cognitive difficulties that result in lower test scores.” I guess so.
Maybe the salt shorts the synapses, or the grease clogs the cerebrum. Maybe it's that kids who eat without much thought take tests without much thought--or that kids who live on burgers and Mexi-fries see their parents less often, and it is the parent involvement that matters.
Maybe I just don't know. I find very difficult to think this issue through, having had tacos for lunch. (Source: seatletimes.com)