Prosecutors had accused Tiny Virginia Makopo of trying to kiss and fondle six girls at the Oprah Winfrey Leadership Academy for Girls.
A South African judge has acquitted Makopo on all charges, saying the prosecution failed to prove their case beyond a reasonable doubt.
Winfrey has issued a statement praising the nine girls who testified at the trial.
"I will forever be proud of the nine girls who testified with the courage and conviction to be heard," Winfrey said in a statement Monday.
The school is spread across a 22-acre campus with neat lawns and garden paths decorated with mosaics. It has computer and science labs, a library and a wellness center. The school is home to about 300 girls from across the country.
The Associated Press contributed to this report.