Bernard Ebbers took the stand at his trial Monday and flatly denied any role in the $11 billion accounting scandal that sank WorldCom, telling jurors his “marks weren’t too good” in school and that he knows very little about accounting.

The former WorldCom CEO said he fancied himself as “coach” of the company and left the numbers-crunching to former finance chief Scott Sullivan. Under questioning from his top lawyer, Mr. Ebbers explicitly refuted earlier testimony from Mr. Sullivan, who claimed that Mr. Ebbers pressured him to cook the books to meet Wall Street expectations.

“I wasn’t advised by Scott Sullivan of anything ever being wrong,” Mr. Ebbers said. “He’s never told me he made an entry that wasn’t right. If he had, we wouldn’t be here today.”

The matter-of-fact denial was perhaps the most dramatic moment to date in the five-week-old trial of Mr. Ebbers, who is accused of orchestrating the accounting fraud that plunged WorldCom into the largest bankruptcy in U.S. history in 2002.

Mr. Ebbers said he has never taken an accounting course and that even as his company grew into a telecommunications giant, he knew little about technology. “I know what I don’t know,” Mr. Ebbers testified. “I don’t, to this day, know technology. I don’t know finance and accounting.”

Mr. Ebbers is charged with fraud, conspiracy and making seven false filings to the Securities and Exchange Commission. The charges carry up to 85 years in prison.