Gamification

Don't worry about what line you're on and focus on good tone quality and make it a game.

I've also been in a situation where of the majority of the class would basically not try at all and do anything they could to derail the rest. Parent contacts just reiterated the issue since they didn't understand why students had to be in band.

Focus on the ones that are trying and give praise whenever something is done well or effort is given. No child has the right to prevent others from learning or take all the focus of the classroom. Be fair and be consistent with all students and focus on the good things that are happening.

Be fair. Be consistent.