MaryAnn Parisi, from Connecticut, America, said she wanted the children bullying her 11-year-old step-son Michael to know "everything he has been through".
The mother posted the open letter alongside two photos of her son - one of him now and one when he was born prematurely at 26 weeks - and challenged readers to share her son's story with the children in their lives.
She wrote: "Maybe knowing his background is the difference. Even the best children have moments of insecurity and weakness."
"Teaching and showing them why he (or anybody else) is different might be the more positive way," Parisi continued.
"Sometimes knowing is learning and growing. Sometimes we all need a reminder, because we all can have our moments."
Parisi explained Michael was born nearly three months premature at 26 weeks and spent the first three months of his life fighting to stay alive.
Three months later, his biological mother left him.
"He has survived failure to thrive and numerous other health issues to become the strong, healthy boy he is," wrote Parisi.
"He didn't learn to talk till he was three years old. Walking was very delayed. He didn't have teeth till after his first birthday. He was so very behind. But he loved. Oh how he loved.
"To this day, his smile is the best thing ever.
"He forgives and honestly, he forgets too. There is not one judgemental bone in his body.
"I strive to be more like him daily but fall very short. You called him brace face today, before you were picking on him because of his eating habits."
The mother went on to explain the difficulties her son still faces now. Difficulties that many who know him know nothing about.
She added: "Did you know he physically can not control the food staying in his mouth or how very bad his hand/eye coordination is?
"Those braces are just one of the many steps he will endure, to help align his lower jaw that never fully developed. So he doesn't spill his food or chew weirdly anymore. So you won't pick on him.
"Kicking his chair, calling him stupid, ugly, brace face, bucky beaver. Telling him to sit down and shut up is not the way.
"You don't have to like him, but you do have to respect him. He's a fighter, that's a very small portion of his story. Share, teach, grow.
"Most importantly respect those around you, you never know what they have been through."
The post, which was uploaded on 12 January, was shared 3,500 times within a week.
Many people commented on the open letter saying how brave Parisi was for sharing her thoughts online.
One person wrote: "This is so touching, my heart goes out to what a special person you are. For such a caring and loving family. Your son is a miracle child... thankfully he was given a miracle family."
Parisi later shared the news that she will be working with her son's school and teachers to end the bullying.
She commented: "[The school] went above and beyond how it was handled and look forward to figuring out to handle this."