Typically, skills and knowledge are divided up into smaller pieces, rather than a year's worth of material.
Because of the flexibility, learning at faster or slower pace than average does not leave overachievers bored and neglected, or force slower students or students whose home life has been disrupted through trauma, divorce or serious illness to repeat whole years to pick up individual skills.
Graded schools being largely an invention of the 19th century, the small, ungraded school could be properly considered traditional education, although they are rare enough now that they are usually classified as alternative education.
Ungraded schools still persist in poor, ethnically disadvantaged, small, rural schools, where the limited number of students and poor attendance make organizing classrooms according to age-based grade levels more complicated.
Further, complete age-mixing of students is considered an integral part of the school environment and culture.