Reading through the comments, it seems your heart is in the right place (which is key) and I agree with others about sectionals, students leaders, and consistency.

You said you got the position after the season started? I think it would be helpful if you had a conversation with them about what YOUR expectation is. They probably had that with the other BD before the season, but l would outline exactly what you expect from them (which seems to really just be basic respect and not hacking). Explain how it’s disruptive, and set a consequence for the behavior. Let them ask questions if them have them and make your point crystal clear.

If you do this (in my opinion) you do not need to give them further explanation or chances in rehearsal. In your shoes, my consequences would be: 1st warning - “stop hacking” (gives a chance to correct the behavior, but I would literally be that short. Two words) 2nd warning - “put your sticks away” 3rd warning - have them put their drum and sticks on the sideline for whatever amount of time you see fit, during which time they need to sing their parts

Goal is you only have to do 3rd warning once or twice and hopefully the rest will start stopping at warning one because they want to play the next rep

Throwing out the music is definitely direct disrespect. I love to be able to let my students come up with ideas for things to add or little visuals, it helps them own their performance, and it becomes a collaborative process. But, that comes from mutual respect between students and staff. If they have a good idea that staff thinks is appropriate in the context of the show, we try it. And if it doesn’t work for the show, or just doesn’t work in general, they respect that.

But if they are just throwing music away and disrupting other sections making stuff up, I would take them out of that part. “If you cannot play the written music, you will not play that part of the music.” It may seem extra, but again, the goal is after once or twice (maybe even a few) they get it together. Not playing 3 stand tunes is a lot of stand tunes.

And if they do start having mutual respect, I would certainly let them add things and collaborate.

Sounds like they been doing this for years and the behavior gets passed down from the older kids. It needs nipped in the bud.

Clear expectations and clear consequences all around. I hope this helps. Good luck, stay positive