A man praised his friend's daughter for standing up against body-shape norms that "persecute children", by refusing to calculate her BMI at school.
The friend, known on Facebook as MacLeod Cartoons, posted photos of the teenager's powerful response about body image positivity on his Facebook.
"So proud of the family of one of my friends," he wrote. "May we all raise strong girls and boys, willing to stand up for themselves."
Having previously been given an "obese" BMI rating earlier in the year, the teenager said she felt self conscious and took herself to the doctor, who told her, she was active and healthy and didn't need to worry about her weight.
"BMI is an outdated way of defining normal weight, underweight, overweight or obese," the teenager explained in answer to the question 'What is BMI?'.
"One of the obvious flaws explained by a men's health weight loss coach is that it has absolutely no way of discriminating fat and muscle.
"It doesn't make sense to me that someone who stays fit, eats healthy and has a low metabolism could be classed as 'obese' and in danger of heart disease?"
She explained the measurement should not be used in a school setting where students are already "self conscious and lacking confidence".
Her school work also required her to calculate her BMI, but instead the teenager wrote about why the calculation affected her so much.
"Ever since I can remember I've been a 'bigger girl' and I'm completely fine with that, I'm strong and powerful," she wrote.
"But I used to wear four bras to cover up my back fat and wrap bandages around my stomach so I would look skinnier.
"My doctor and I talked about my diet and how active I am. He told me I was fine.
"I am just beginning to love my body like I should and I'm not going to let some outdated calculator tell me I'm obese."
She added to the end of her answer: "My body and BMI are perfect and beautiful just the way they are."
The photos of her work were uploaded on 30 March 2016.