This way, if something is easy or interesting...or tough or boring, kids can choose how they want to tackle it. Having many different ages in the same class without the pressure to finish everything crammed within one year should help. Here is where maybe bright kids can teach kids that need help..or older kids can help younger ones.
Example: ages 5-10 study together in huge single room schoolhouses ..each one on whatever they want to learn. Ages 11-15 is another group etc. amongst them, they can be in sub groups according to interest or ability. A class can have 4-5 teachers who can tackle all of the subjects. Volunteer parents.
Test and grade them at the end of five years. Only test them every year or semester.