A Mansfield Woodhouse mum said she was horrified to discover her four-year-old son alone outside the school grounds when she went to pick him up.
Brambles Academy has launched an investigation into how little Joshua Spencer managed to evade teachers and a receptionist and reach the street outside.
Melissa Spencer told how she arrived at the academy on Friday March 4 to pick up her son at 1pm
She said: “I had gone to pick up Joshua as usual at 1pm and I was waiting in reception.
Two pupils came running up to me and said my son was outside the school gate which is controlled by staff.
“I was horrified and aced up to and sure enough there he was - on his own next to the main road with no teachers or anyone else from the school in sight.
“Anything could have happened to Joshua.
“I was distraught - I thought he was safe inside school and clearly he wasn’t
“I was an absolute wreck, I had to go to my mum’s house I was in such a shock.
“I couldn’t understand what had happened I thought you couldn’t open the door without a member of staff opening it for you.
”All I can think of is when he was playing on the yard a teacher had taken their eye off the ball.
“Nobody knew he had gone and there was no follow up - he had been missing for five or six minutes.”
A spokesman for the school said: “We are aware that last week a child left the school grounds during the lunch-time period.
The Bramble Academy takes the safety and wellbeing of all our pupils very seriously and have launched a formal investigation into this incident.
“As the investigation is ongoing, it would be inappropriate for us to comment any further at this stage, but we would like to assure the community that we are treating this incident extremely seriously and following the outcome of the investigation will put any recommended measures in place to prevent incidents such as this happening in the future.”