Staff only discovered Alfie was missing when his mum Natasha went to collect him from his playgroup on Friday.
Gripped with panic, she found him a few minutes later looking lost and confused in a road nearby. The mum said she feared he could have easily been snatched by a paedophile.
She said she now fears for his safety and is reluctant to let him return to Robinswood Primary School in Gloucester.
Head teacher Martin Latham admitted: "It was a mistake and it shouldn't have happened."
Natasha, said she wept at the roadside as she comforted her son on Friday.
She said: "I couldn't believe what I was seeing. My heart was in my throat. Alfie could have been kidnapped or killed. I couldn't help but think of little April Jones.
"I was crying my eyes out and he was very confused so I just cuddled him.
"I asked him what he was doing there and he said that his teacher had let him go."
Four members of staff were present at the pre-school, but Alfie managed to bypass them all.
The school installed a new security gate at the weekend and Mr Latham held talks with the mother yesterday.
He said: "We have a clear system of how the children leave the pre-school. We thought we had a secure system but something went wrong.
"It can be quite congested at home time and I think he had got out around the back. "We have looked into it straight away and over the weekend we installed a new gate.
"It is not an excuse and we hold our hands up to it - it is ultimately my responsibility."
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