Simplify the following text for a Grade 6 native English-speaking person.
Original: Advocated by groups such as the National Middle School Association , the middle school concept is a relatively new model for the middle-level grades ( 6-8 ) , contrasted with the more traditional junior high concept ( 7-9 ) .
Simplified: Depending on location , middle school contains grades 6-8 , 7-8 , or 7-9 . Middle school is also sometimes called junior high school or just junior high .