Whether it's a specific class that is always challenging, or it's a time of year that is particularly difficult, finding lesson plans that will work for those challenging days can be a struggle. The specifics will vary depending on the exact situation and group of students, but here's what I look for when I'm planning for a lesson that I know is going to be difficult for one reason or another.
1. Less talking, more doing
The less time you're spending trying to get students to listen to you the better! Two decades in and I can still use the reminder every now and then that we DO NOT have to verbally explain nearly as much as we usually do- just jump into a song and they will follow. I look for lesson activities that don't require much explanation, whether that's something they can learn by following my movements, echoing my singing/ playing, or putting up notation on the board, pointing to the notes, and saying 1, 2, ready, go!
2. Get out of the way
This is similar to my first point, but I try to look for activities that involve as little "follow the teacher's directions" as possible. Maybe they are following directions from a video, having a student(s) lead the activity, or working in small groups or independently.
3. Lean into the right brain
This is especially for those times of year when everyone is a little more drained, like right before a break: I want to engage students, but not expect a lot of critical thinking. I've found that engaging their creative ideas, not their analytical thoughts, works best in those moments. It gets them cognitively engaged with a part of their brain we don't ask them to use as much at school, which feels exciting and fun.
4. One thing, a hundred ways
The most successful lesson plans in particularly challenging situations are the ones where I can read the room and instantly shift gears. If I have a group or a situation where I'm not exactly sure how they'll respond to an activity, or how quickly they will grasp a new concept, I plan several different activities to address the same concept or skill and have all of them ready. If the first one I try starts to feel like I'm pulling teeth, I can move on to something different knowing they'll still learn the same thing. It also focuses students' mental energy on one thing, so even if I do end up doing all the activities I planned it feels more manageable for students and they're more likely to be successful.
5. Play to their strengths
If I know a particular group is most successful at, or most interested in, a specific type of activity, I incorporate it as much as I can. Sometimes I have a group of students that loves to just sing. Often it's playing instruments, or especially with younger grades, it's dancing and movement. Sometimes if the group dynamics are challenging, they do best with small group and independent work. I certainly don't abandon other forms of music-making but especially if I know a new concept is going to be difficult, or I know a specific day is going to be challenging, I'll try to incorporate the ones that they're most successful with and use them to teach whatever it is I want them to learn.
I hope this helps give you some ideas of what to look for when you're dreading a particularly difficult day or specific group of students. It can make such a huge difference for both the students and for ourselves as teachers when more students are excited about the lesson and walk away feeling successful!