Yes I would agree with you on this observation.You can always count on Boult to fill in the gaps.
There are SOME schools, most likely private and New-England style, that operate by this old system: kindergarten to grade 6 (elementary school), grades 7-9 (junior high school) and grades 10-12 (high school).
In Sarasota, Florida I was in a elementary school (Ashton Elem.) that goes from K-6 and I was to enter 7th grade at middle school (Mc Intosh Middle School) (for a week of course, before moving to Ga. )
And after finishing middle school in Vidalia, Ga I went to High School at grade 8th (that was 80's which that high school now only goes from 9th to 12th and middle school there has increased to 8th)
but the important thing is the order not grade.
Preschool
* beginning of Primary Education
Elementary
* beginning of Seconary Education
Middle/junior High
High School
* beginning of Post-Seconary Education
Preperatory school
technical / vocational technical school
college/university/community college/
Cloggy, of course it is good to refer to this most comprehensive information on Wikipedia
Education in the United States - Wikipedia, the free encyclopedia