Mr Jenkins, 40, told how everything seemed unreal when he was confronted with the girl's blood-soaked body on the patio of his house.
"From the moment I used my hands to touch her hair, an elephant could have been in the house and I wouldn't have noticed it, I was so shocked," the deputy headmaster said, in a statement read out at Lewes Crown Court. "Everything just seemed so unreal. I was waiting for everything to end. It was like a bad dream."
Mr Jenkins said he was in despair. "I was expecting someone to come in in the morning and say hello and everything would return to normal."
Mr Jenkins is accused of murdering Billie-Jo, whom he and his wife, Lois, had fostered for five years, at their home in Hastings, East Sussex. He denies the charge.
In four days of interviews with Detective Constable Steven Hutt, Mr Jenkins told how the body of Billie-Jo was discovered by his 10-year-old natural daughter, Lottie, who screamed "dad" when they returned from her clarinet lesson in February last year.
Billie-Jo was lying on the patio where she had been painting the French windows and had been beaten about her head with an 18in metal tent peg.
Mr Jenkins said: "I crouched down and was looking at Billie. I saw lots of blood all around her head. The blood was thick, it didn't really look like blood, although it was."
After taking Lottie and another daughter, Annie, then 12, away from the scene, Mr Jenkins told police that he returned to Billie-Jo.
He said there was a squelching sound when he moved her shoulder and her head was limp. "I knew by this time that what had happened to Billie could not have been an accident."
But he said he spoke to her, reassuring her that people were coming to help. He then described how he noticed her forehead was misshapen and that she had a swollen eye as if she had been punched. A bubble of blood came out of her nose. "I believe she was alive at that moment in time," Mr Jenkins told police.
He said he had left the 13-year-old girl painting because she wanted to do some chores to earn extra pocket money.
He had earlier shown her what to do and told her off when she painted the inside of the doors against his instructions. She had stuck her tongue out at him "in jest".
Mr Jenkins said he told her how to spread the paint and added: "I expected Billie to make a mess of it because I knew she was not a natural decorator. Billie was quite an impatient girl, but I wanted to give her a chance."
A short while later, he again noticed she was failing to do the painting neatly and crouched down himself to show her what to do. He described how, in horseplay, she had mounted his back and put her legs around his shoulders as he tried to paint. After climbing off, she asked if she was not doing the job properly.
"I went over and cuddled her and said, 'of course you are'. She was the kind of girl who needed reassurance," Mr Jenkins said.
When Billie-Jo had joined the Jenkins family, her father had been in prison and she had spent a short time in care, the court heard. But she had adjusted to her new life and grown more confident, Mr Jenkins said.
The trial continues.Reuse content