system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are elementary schools so much smaller than middle schools, than high schools, etc?

In the area where I live in the US which I'm going to assume is generally emblematic of most areas in the US there are a ton of small elementary schools, often really close to each other, some middle schools, and a handful of high schools across the entire state. 

Why are elementary schools made to be small and yet densely packed in an area compared to a high school?
gpt
Several factors together:

Little kids tend to need more individual attention (smaller student:teacher ratios), so you need more classes at the elementary level. That means you need more classrooms. You could just stuff them all into one bigger building but...

Little kids don't do as well with long bus/drives, and they can't drive themselves. It's a lot more convenient for middle/high schoolers to walk/bike/drive/bus (far) to school than little kids. So you want the elementary schools close(er) to the students. That means a big number of relatively small buildings spread around, rather than one huge building.

And middle/high schools tend to use much larger/specialized/expensive resources, so you want to concentrate people to make best use of them. You're not going to build a 400m competition track, pool, mid-grate theater, full chemistry lab, machine shop, etc. at every elementary school.