Nancy Boyd calmly described what she saw on a video feed.
Boyd told the 911 dispatcher "there's some man here with a gun."
At one point, Duke fired shots wildly at board members.
Security guard Mike Jones rushed in and shot Duke, then Duke shot himself.
"Being a law enforcement officer, you always think about this day and I just want to let the Duke family know that my heart goes out to you," Jones said.
Duke was dead by the end of the 911 call.
"He loved me, he loved his family," said Duke's wife, Rebecca. "He was just trying to get others to stop, as he would say, dumping on me."
Duke's wife said he was an excellent marksman and probably missed on purpose.
She said he was distraught after she lost her job with the district.
The Associated Press contributed to this report.