1. With the large group, do lost of playing and only give them two things to make better each rep. Keep your instructions short. Horn in mouth = not talking. They will get tired, but your are building endurance and solving a problem. When they can take more instruction give them one more thing to thing about. Two individual goals and one ensemble, two ensemble and one individual. One listening, one individual and one ensemble. Never give more than two or three. If you want to do more, then just do more reps with different goals each time.
2. Don't acknowledge the late kids in front of the group. Make them see you after rehearsal and hold them accountable. Follow up with a phone call to the parent. Make it as uncomfortable as possible within policy. Ultimately, next year write for less people and take them out of the show. Real culture change comes from the kids. It will take you getting a leadership team that understands your expectations and helps change the culture of the band. It can happen quickly, but it will take some time. You could give all the kids that show up on time something. Donuts after rehearsal. Let them out a little early on Friday. Again, reward the good behavior. Be careful to keep the reward in line with what you want.
3. Understand that the Seniors are not yours. Kids don't understand that you had nothing to do with the previous directors departure. They just know it isn't the same, not what they expected. It's really not fair to them. "It's not good, it's not bad, IT JUST IS." That's for you and for them. Make it clear that you did choose them. You made a decision to come and be their director. This changes the perspective of the kids. Have a meeting with the Seniors. Explain the previous statement. Help them understand that they are not doing anything, but hurting themselves. They control the quality of their experience through their attitude and approach to their role in the program. You can maximize it or minimize it. Either way we all win.
4. Welcome to big band! More kids = more issues and excuses. That is why the culture of the program is so important. Work with your leadership team to change the culture. Make it to where the kids don't want to miss, because of how fun it is. Of course, you have to define fun. Having high standards and getting better is fun! Getting better, together is fun! Don't focus on the wrong kids. The ones that miss. Keep your focus on the kids that are there. Over time, those kids will get on board or start to feel uncomfortable and leave. If students miss a performance, make them write a paper. The assignment must be made up. Hold them accountable.
Keep your head down and keep working toward your goals. You are certainly going to make things better, because you are asking the right questions!
Repost in a year and then the year after with what worked for you!