You're making some pretty broad generalizations here.
I have been overweight/obese my whole life. I've also had a considerably higher than normal IQ my entire life.
If I ever acted lazy or 'stupid' at school it was because my self image was effecting my drive. Being fat didn't make me either of those things. It certainly didn't effect my intelligence.
I have always done well in school because I have always felt like I had something to prove. Before it was that the fat chick could do it... now it's that the 29 year old chick with two kids can do it
My eating habits were deficient, not my intelligence quotient