Why did Ash need to go back to Pokemon School? I was watching Pokemon Sun Moon. In the answer to this question "Why did Ash became a child back in Sun Moon Series?" it says that Ash never age so obviously he didn't became child.

But why did he need to go to Poke-school? He knows a lot on Pokemon and he went on till the finals in last league (Kalos)
As I said in the comments, Pokemon is a series which has been on air for a long time. But it's still mostly aimed for kids.

Whereas kids watching the first ones are now adults, it would be strange to have a Ash kid, pro at Pokemons and already knowing everything.

So at each beginning of a new series, they explain everything again for the new kids watching and because it's important to see a progress. If Ash knew already everything, had all the Pokemons from various regions, then it would become boring to follow him on his journey as he will certainly be a lot stronger than most of his opponents.

By doing as if Ash knew nothing at the beginning of each series, you can then follow again is evolution and grow up with him.

(Sorry for bad English as I'm French, and hope that you still understand)