It's a very flexible term, and generally comes down to what a local school district says it is. Nominally, "Junior High" is seventh, eighth and ninth grade, but when mixed with the term "Middle...
junior high is 2 years, high school is 4. high school also matters much more especially sophomore junior and senior year because these are the main years that colleges look at. high school also...
Nothing comes after "Junior." "Junior" and "Senior" are a related pair describing a father and son, uncle and nephew, etc.
Relatives with the same name may also be numbered, as James Smith I, James...