How does junk food affect a teen's ability to focus?

Snacking on junk foods I could get at the cafeteria or school co-op left me chronically tired and cranky, making it difficult to concentrate on what was being taught. I found out later that my diet of simple carbs (which are basically just sugars) contributed to my inability to focus by making me jittery and hungry all the time. These foods don't have enough fiber or fat to keep you full for very long. If you're always hungry and thinking about your next sugar-and-carb fix, of course you're going to be distracted.