One word: GROUPS!! Have them sit according to grade level (assigned seats from Day 1).
Differentiate each assignment for each grade level.
Example: Composition assignment. 8th grade: compose 12 measures of rhythm and melody which must include (man I wish I could draw!) at least two each- quarter, eighth, half and whole notes and rests. 7th grade: compose 8 measures of rhythm and melody which must include one each of [same note /rest values as 8th grade]. 6th grade: 6 measures of melody and rhythm which must include one each [same note values] – no rests.
Teach topics as a whole group (orchestra / band instruments, composers) Break them into grade-specific groups and have them do projects / research and present to the entire class. Ensure that everyone understands how to be a respectful and supportive audience — not normal for this age group.
Middle Schoolers are always reluctant to sing, but don’t shy away from this. Have them sing in at least two parts. You easily have three parts – grade levels! Have them sing in unison and listen for the lowest / highest voices. separate them. 6th grade boys voices are not usually low enough for tenor, but 7th or 8th should be. Give them simple vocal ostinati at first, combined with two higher vocal parts made up of the rest of the group.
*** Never have middle school kids sing solo unless they tell you that they want to / are comfortable with it!!! Most are extremely sensitive and insecure at this age. Gotta treat them like babies while pretending to treat them like adults.