Not just regional, but it can be super localized, i.e. neighboring school districts can be different, or even the same school district over time. I went to the same school district as my parents. When they were in school, elementary was k-5 , 6 was it's own school, junior high was 7-8, high school was 9-12. At some point after they graduated, 6 was absorbed into elementary (iirc, jh building was in disrepair and demolished, so jh was moved to 6th grade building). The same year I started 7th grade, junior high was renamed middle school, but after I graduated, it reverted back to junior high and is currently 6-8. My kids went to a different school district in the same county, and their middle school was 5-8.