I have this discussion several times a year with my students.
Most students believe that band class (or choir or orchestra) is the time for them to learn their music. To me, this is like the offense for the football team showing up to practice and having no clue what their assignments are for any play. Football players are expected to know the plays before practice begins, and I believe that music students should know their music prior to rehearsing with the band. Class is where they learn everyone else’s part.
Band directors need to work on phrasing, balance, interpretation, and style; they don’t have time to teach students their individual part. By coming to class prepared, the band director can work on shaping the music, the student can hear how their part fits in with the rest of the band, and the rest of the band can hear how their part fits in with them. This is the essence of rehearsing.