Apparently he was engrossed in whatever he was playing and didn't notice them line up to go into the playground!
He didn't know what to do when he realised everyone had gone, but sensibly decided to stay put till they came back.
I am pleased that he had the good sense not to wander off, but shocked that no-one noticed he was missing until they lined up to come back in from the playground.
He only started full-time on Monday and fortunately this 'incident' doesn't seem to have worried him too much but I am quite concerned that no-one noticed he was missing and as a result he was left unsupervised for a time.
I shall obviously be talking to his teacher in the morning, but just wanted to check that I'm not being overprotective first (am pregnant and hormonal at the moment).