The trial is set to start March 29, The Associated Press reports.
Winfrey's lawyers say in recent filings she must attend as a named defendant and has rearranged her TV production schedule to do so. She also appears likely to testify.
The case centers on remarks Winfrey made in suspending a headmistress amid the 2007 abuse complaints.
Winfrey had said she "lost confidence" in Nomvuyo Mzamane and was "cleaning house from top to bottom."
A judge this week refused to drop Mzamane's defamation suit. A dorm matron was later charged with abusing six students.