I think its an age thing too
Do the teachers keep an eye on them after school? Maybe you could have a word to them about him waiting for you inside the gate/where ever & them chasing him up.
Most schools have a one gate rule & our last had a teacher on gate duty & kids weren't allowed to go til they'd been mentally marked off & sent with adult/bus/older child. However that child usually went home.
But then if he nicks off to the playground there wouldn't be much they can do!
Maybe give him a book or something specially to read while he is waiting. If he is in Kinder he's probably confused with the time of day too. Outside = playtime!
He should get used to it soon. It took DD1 a while