Shouldn't the really young kids at elementary schools be separated from the older kids?

I went to a Catholic grade school which was K-8, and IIRC only kindergarten was treated separately. AFAIK mostly because they wanted more supervision of little kids in their first year of “real” school.

My elementary school was K-6. Except in kindergarten, we weren’t separated from the older kids and we didn’t get let out any sooner.

That’s how my school worked too; each grade level was more or less self-contained (except for G&T classes; those had multiple grades at the same time), but walking to and from school, or riding the bus was combined.

99% of fighting/bullying/interaction with older kids was walking to or from school. If an older kid rode the bus, I never even interacted with them in elementary school.

In middle and high school, we had fixed class periods, and classes that were multi-grade, so there was a lot more interaction with older/younger students beyond just what we had going to/from school.