I'd almost never heard middle school but it was changing right after I was in school. Middle schools or middle years have been replacing junior high schools in the shift from two-tier to three-tier education. Japanese and Korean 中学校 are more like junior high schools because together junior and senior high schools have the same number of years as elementary/primary/first/early-years schools. I'm surprised to see Duo in the notes translating 初(초) as early as it's always meant first/beginning/onset to me . . .