There have been discussions about whether school attendance should be extended or shortened. It makes sense to start from the end! This means starting with what graduates should know and be able to do.
Once we have established this, we can set requirements for universities (through an accreditation committee). Universities would then logically be able to say what students should know when they start their course.
These requirements are set for high school graduates, which in turn determines what children should know by the end of elementary school.
Only then can we discuss whether children can learn this in five, eight or ten years.
It is pointless to extend or shorten the school system without knowing what its goal is.