Leia Rochelle Pierce from Denver, Colorado, said her son, Jamel Myles, had been proud to tell people he was gay when he returned to school.
She told Victoria Derbyshire some fellow pupils told him to kill himself.
Denver Public Schools said it was committed to ensuring all pupils were treated with dignity and respect.
Ms Pierce said she blamed herself for his death, adding that her “heart was breaking every second”.
“I feel responsible because I didn’t see the pain in my baby’s eyes. I should have just known. It’s my fault, I didn’t know.”
‘Proud of who he was’
Ms Pierce said Jamel had told her that he was gay over the summer. He had not planned to formally come out at school, but said he was “proud of who he was and he didn’t mind telling people”.
She said Jamel had seemed normal after his return to school, playing on her laptop and watching television as usual. But last Thursday, after only four days of term, she found him dead.
“I’m pretty sure he told someone who got that whole persona of ‘that’s not OK’ and decided to pick on him,” she said. “I’ve sat here and seen kids pick on kids for less. I’m pretty sure he told one person and it spread and it became a worse situation.”
Ms Pierce said after he died, her eldest daughter said Jamel had told her he was being bullied because he was gay.
“My son came home and told her the kids at school were telling him to kill himself,” she said. “He didn’t come to me and that hurts as I would have understood and I would have defended him. They were so close, he would just tell her everything.
“I don’t think it’s fair to go through it. That image, seeing him like that, is burnt into my brain and it hurts. I never wanted to see my baby like that or for anyone to see their baby like that, it’s not right,” she added.
‘Beautiful and special’
Ms Pierce says all young people, regardless of their sexuality, should feel special, welcomed and warm.
“I would tell them they’re beautiful and they’re special and there’s nothing different about them that should be pointed out and make them feel anything other than loved,” she said.
“We are all different and it’s our differences that make us equal, because it’s the one thing we all have in common.”
Ms Pierce said Jamel’s school had told her in a phone call it was going to work on suicide prevention. She said it needed to stop bullying, which was what was leading to suicide.
“Teach your kids love, teach them it’s OK to have differences, we’re all different,” she said. “Teach them compassion, teach them respect, teach them to be more accepting of each other.
“It doesn’t matter if you’re the bully or you’re not the bully, there’s pain in everybody. Until we correct the pain and the hurt that’s in everybody and turn it into love, nothing will change. We have to change ourselves for our children.”
Ms Pierce said Jamel “was magic”. “This little boy could walk into any room and make any person feel so loved and so special. He had this pizzazz about him.
“He wanted to make a change in this world and he wanted to show people love. And he can’t speak right now so I am speaking words he said for everyone to hear, because a gentle kind soul just left this world because of something so cruel. And I want my son to know he made a change for the better.”
Denver Public Schools said it was “deeply committed to ensuring that all members of our school community are treated with dignity and respect, regardless of sexual orientation, gender identity or transgender status”.
In a statement, it added: “Our policies and practices reflect this commitment to ensuring that our LGBT students can pursue their education with dignity and joy. We also know, however, that we as a society have a long way to go to ensure that no child ever is bullied or treated with disrespect because of their self-identification.”