that's impractical, unfortunately (Score 3, Informative)

Yes, but it will remain that way for largely practical reasons. Having high schools let out earlier than elementary schools means that the older kids will be home to look after their younger siblings. The busing costs for school districts would skyrocket if all school levels ran simultaneously. The bus drivers first take the high school kids, then the middle school kids, then the elementary school kids. A school district would need a whole lot more buses running simultaneously to get everyone aged 5-18 to school at the same time. My high school and elementary/middle schools were actually two different districts, because I went to a regional high school. They had a bus-sharing scheme like this one worked out among all the districts.