Although I would doubt he was beaten in school, I would bet that in Primary School he was picked on for his weight. Kids are cruel and although JK makes a point of saying how he picks on Harry, she does not say he was the school bully then (not until Smeltings). My guess is one of the reasons he picked on Harry so much is to raise his sense of self-worth. Most kids jeered himn for his size and weight so he picked on Harry to make himself feel better.