It was Wednesday, March 13, 1996.
Arriving at the school at around 8.50am she dropped off her children in the playground, where others were saying goodbye to their parents. She then headed for the gym and spent a few minutes chatting with colleagues. An assembly for 250 children and adults, led by the local chaplain and headteacher Ron Taylor, was under way in the hall as she began setting up the equipment for her first lesson.
The lesson, with Gwen Mayor's primary 1 class, began shortly after 9.30, a few minutes late because the 28 primary pupils, aged five and six, had been to assembly.
Mary Blake, a supervisory assistant who worked with special needs children, was with them. Mrs Mayor told her she could only stay a few minutes as she had been called to a meeting.
As usual, the lesson began with the children running around the gym to warm up. Mrs Harrild remembers taking a pair of glasses from one little boy and placing them on Mrs Mayor's diary for safety. It was her last recollection of an ordinary day.
It was 9.37am. "I called the children to me. Then I was aware of the door being opened suddenly and I turned to speak as it was quite normal for people to come into the gym," she said. "But as I turned, I saw a man wearing a woolly hat. He had a gun extended and immediately began shooting."
Without saying a word, Hamilton - wearing earmuffs and glasses, with four gun holsters strapped to his body and ammunition over his shoulder - fired at the teachers first. Mrs Harrild was hit in the arms and chest, and Mrs Blake in the head and legs. Mrs Mayor suffered fatal injuries.
The killer then turned to the terrified children, gunning them down with his automatic Browning pistol. At some point during the slaughter, he walked up to where the helpless injured lay, stood over the tiny bodies and shot them one by one, at point-blank range.
By this time, Mrs Harrild and Mrs Blake were huddled with four or five injured children in the gym's open store area, where they had scrambled for cover.
One child kept saying: "What a bad man."
In less than four minutes, 105 shots had been fired, 17 people, including Hamilton, were dead and one child was dying. Eleven children and two adults were injured. Only one boy escaped unhurt, crouched behind his dead friend.
At the other side of the school, headteacher Ron Taylor, 46, was on the telephone when he heard "banging" noises. He recalls feeling slightly irritated, because he thought builders were in the school and no one had informed him.
Mr Taylor's office is by the main entrance, but Hamilton had slipped into the school unnoticed via a side door next to the boys' toilets and gym - one of six entrances to the school.
It was not until assistant head, Agnes Awlson, 47, rushed into his room and told him there was a gunman in the school, that he realised something was seriously wrong.
She had been taking her class to an art lesson in the main building when she heard "metallic sounds" and distant screams. On approaching the gym, she saw spent cartridges lying by the door.
Mr Taylor immediately rang 999 then dashed to the gym, unprepared for the horrific scene he would encounter there. On the way he passed a wounded supply teacher in a corridor being tended by Mrs Awlson.
"I burst into the gym. It was a scene of unimaginable carnage. It was one's worst nightmare. The air seemed to be thick with bluish smoke and the smell of cordite was quite strong," he said.
Within minutes other teachers and ancillary staff were tending the injured and dying.
At one point Mr Taylor thought he saw Hamilton move and ordered the janitor to kick the gun away from his hand.
Police used curtains from the school stage to cover the gym windows, as primary staff and police began the harrowing task of identifying the tiny bodies.
Some had up to seven gunshot wounds. The difficulties of identification were compounded because the register had not been taken that morning, and photographs on the children's record cards were "of little use", Mr Taylor said.
By 10.30am more than 300 frantic parents had gathered outside the school, anxiety etched on every face, as the news emerged in dribs and drabs.
At first it was thought 12 were dead, but the number kept rising. Many had heard of the tragedy on the radio.
Across the city, Dunblane high came to a standstill, as teachers and pupils, many with children or siblings at the primary, struggled to come to terms with the tragedy.
It was not until around 11am that Mrs Mayor's class were firmly identified as the victims of the murderous assault, and by then many of the injured were already on their way to Stirling Royal Infirmary where beds had been cleared for the emergency. Affected parents were taken to two nearby houses and then up to the staffroom, where the tragic news was broken. But confusion about the children's identities meant that some parents had to wait until 3.30pm to hear their child was dead.