The girl at a secondary school in Naduang district of Loei province used a nylon rope to hang herself from a beam inside her bedroom.
A few hours before committing suicide, she posted on her Facebook saying last goodbye to her friends for personal affairs and distress.
However friends said they thought she was just kidding.
Her name was withheld by police as she was still under 15-year-old.
Her parents were not suspicious of her death because she had earlier complained of her distress in life to relatives.
Girl takes own life after sending last goodbye to friends on Facebook - Thai PBS English News