Have you ever heard the phrase "garbage in garbage out"? You can't expect your brain to function at full capacity if you've been eating mostly junk food. Protein is great for providing energy for both the muscles and brain, but of course you want lots of fruits and vegetables and some carbs as well. I've found personally that consuming candy or other things high in sugar lead to myself behaving more impulsively, usually leading to blowing off schoolwork/procrastinating. I recommend that you experiment with slight tweaks in your diet and really pay attention to how your mind and body feel and function as a result.