Behaving differently in school and at home

Hi everyone, I'm a mummy of two lovely sons and currently pregnant with my third. I'm just wondering why my elder son (age 4) can do so well in school where all the teachers are complimenting him, but at home, he will tend to do everything the opposite. Example, knowing that it's not right to hit his younger brother (age 3) but still does it in front of me and walk away smiling. Can anyone tell me why?