Why the separation of schools for grades K-5/6-9/10-12?

Question

Why the majority of the world not just use a single school from K-12 in general? Is it due to kids growth or teacher specializations?

In: 1

Answer 1

Most schools are grouped by age as the populations are too large, and having multiple k-12 schools is more costly than elementary, middle, and HS being separate. Also, the ages are so that students are grouped similarly- cuts down on bullying and avoids things like a 15 yo trying to be “friends” with a 7 year old.