I've seen this before and think it's great. Of course we can't control what happens in school and on social media but it all starts in the home. If you grow up with your mum telling you you're fat and/or constantly at you about you weight, it doesn't matter what your friends say or anyone else says because you've grown up with a bad self image from day one - and that I believe is much harder to repair (impossible even).
I can distinctly remember my mum jokingly calling me porky as a 4 year old. Although I was never overweight I wasn't wiry like my cousins and I was constantly reminded of this by my mum and aunts. 4 years old!! This continued throughout my childhood, escalated during puberty (my mum acted disgusted that I'd gained weight and 'ruined' my body). I am extremely surprised I didn't end up anorexic or bulimic. I was always tall and athletic but thought I was huge thanks to the body image I'd grown up with. I still struggle now even though I know I am fit & healthy, I don't believe people when they tell me I am slim.
I really hope that if I have a girl, I can encourage a positive body image but I am also fearful as I can't shelter her from others. I will be giving both my mother and MIL strict instructions not to ever mention body size or shape in front of my daughter. It comes from extended family too!