GANGJIN COUNTY, South Korea — Every morning on her way to school, Hwang Wol-geum, a first grader, rides the same yellow bus as three of her family members: One is a kindergartner, another a third grader and the other a fifth grader.

Ms. Hwang is 70 — and her schoolmates are her grandchildren.

Illiterate all her life, she remembers hiding behind a tree and weeping as she saw her friends trot off to school six decades ago. While other village children learned to read and write, she stayed home, tending pigs, collecting firewood and looking after younger siblings. She later raised six children of her own, sending all of them to high school or college.

Yet it always pained her that she couldn’t do what other mothers did.

“Writing letters to my children, that’s what I dreamed of the most,” Ms. Hwang said.

Help came unexpectedly this year from the local school that was running out of school-age children and was desperate to fill its classrooms with students.