The sex occurred with a 15-year-old in her car and twice at her home between September and November 2005, according to arrest warrants.
Robbins was a private school teacher at the time, and officials said the accusations did not involve one of her students.
Robbins was released from jail on a $90,000 bail.
Messages left at a telephone listing for a Karen Robbins in Clinton and with the county public defender's office were not immediately returned.
Police said they began investigating after school officials turned over letters in July found at Bell Street Middle School, where Robbins worked during the 2006-2007 school year.
Earlier this month, Allena Ward, also a former Bell Street Middle teacher, pleaded guilty to having sex with five teenage boys, some of them students where she taught.
Wendie Ann Schweikert, a former elementary school teacher in Laurens County, was sentenced in June to 10 years in prison after pleading guilty to having sex with an 11-year-old student. Ward has not yet been sentenced.
"You're concerned as a parent. You're concerned as a citizen and concerned as a prosecutor that these events keep being uncovered," said solicitor Jerry Peace, who prosecuted Ward and Schweikert.
The now-17-year-old boy in Robbins' case, a former student at Bell Street Middle, is cooperating with authorities, Peace said.