Why is it forbidden to teach the basics?

I hear that it is forbidden for teachers to teach anything taught in the previous grade at school in Japan.
So they must be forced, for example, to teach equations to those who don't understand fractions, to teach about European Union to those who don't know where Europe is or whatever.
I wonder if it is for the purpose of teaching them that they can't rewind or reset their own lives :-P

The Turkish Version