Candice Curry, from the US, explained that she needed her daughter to leave school to get her brother, but assumed she would just take him home.
“Instead of missing class she actually took her three-year-old brother back to high school with her and let him sleep on her while she finished her last class of the day,” Curry shared on the Love What Matters Facebook page on 13 August.
The mum continued: “I’m not sure on the legalities here or what kind of rules were broken but I also just don’t care.
“My sweet teenager helped her family out and returned to her other responsibility while snuggling her baby brother.”
The photo showed the three-year-old boy fast asleep on his sister’s lap while she was sat in class. The caption read: “Bring your kid to school”.
Curry wrote: “I’m either the worst mum in the world or totally nailing it. Please don’t tell me which one - let me live in ignorant bliss while I stare at this picture.
“Life is short. These are the moments that make it so incredibly sweet.”
The post has had 100,000 likes and many parents were astounded that the teenager got her three-year-old brother to actually take a nap.
“Everybody is just going to ignore the fact she got a three-year-old little boy to take a nap?” someone wrote.
Another replied: “I know right? I’m currently listening to my three-year-old kick and scream about a nap. Is she available to babysit my kids?”
Curry replied: “Haha. He was exhausted. She’s the toddler whisperer.”