1/ Dividing kids in age based groups is very poor. They only learn negative behaviour from each other. Skill based would be more interesting as the younger kids would learn from the older students and the older ones learn some responsibility towards the older students
2/ Scientific research has proven teacher based education to be sub-optimal. You take away the inquisitive nature of the kids. They will take what the adult says as true and fail to look for different ways to reach the same goal.
3/ School just isn't fun. Information is pumped into children which they forget once it has been tested. Let them discover things in a playful way and they will remember it much longer.
4/ Exams really only test how well one can game exams and tests, not how well one understood the matter. Test should be used as a personal measure to check if you understood everything and if you are ready to move on to more complex issues, not as a benchmark compared to others.
And so much more.