well, neither of these are real reasons to have the proper nutrition. eating well can improve your focus, which can help with your grade average, and yes i understand the want to be skinnier, but these arent the main reason to be on the proper "diet". getting in a ruetine of eating healthy foods is a good thing. it has many medical benefits. also eating healthy can reduse your stress level. which can also improve grades. if these are things that you are concerned about, then i would suggest eating proteins and avoid most starches. fruits are great natural sugars.
hope this helps ;)