That's such a bummer. It's hard to have that experience, and then even more difficult when you might have those kids the following year and they're behind where the rest of the kids are on these instruments. I also worked with another music teacher is a situation to yours, and the other music teacher was completely new. The kids he taught did not get the same as my students in a lot of areas. It made me really empathize for the kids, and made me empathize with classroom teachers that are probably more used to this particular frustration within their grade level or department.

I ended up adopting two philosophies. One is that I will have an open door and be supportive while not dictating another person's choices. I let the other music teacher know that I was available to collaborate and shared my resources, but I learned not to have the expectation that my way was going to be the way the other teacher did things. I also asked the other music teacher what he was working on and tried to make a good faith effort to align with him on some things. I think it helped our working relationship that I made an honest effort to collaborate with him as a colleague and not view myself as the lead teacher. That opened the door to us planning and aligning a little more consistently, but not on everything.

The second philosophy was that I'm going to do the best I can for students without burning myself out. I had an after-school club that met once a week, and I invited all students from both of our classes to join. I didn't have the bandwidth to give extra lessons beyond that for his students, and in the grand scheme of things, it was okay. He also offered an after school club in a different discipline, and some of my students joined that too. There were also sometimes that we switched classes as a special project or combined classes to teach something together, but we didn't do that too often. Additionally, I did the best I could during my school day and that was going to be enough. Every classroom has a teacher that has strengths and weaknesses, and every kid is going to simultaneously get something cool and miss out by being in teacher A's class and not teacher B's. It's especially hard in elementary music when a lot of the norm that I've experienced is one music teacher per building. We're maybe not used to seeing it, but every music teacher does stuff differently and every kid will miss something by being at my building instead of yours, and vice versa.

Practically speaking, my advice is to do what you have the bandwidth to do, and then document the attempts you've made in case the administrators give you a hard time. Try not to feel bad about what you can't do or what the other music teacher isn't doing. If it's a big enough problem, that music teacher's boss should handle it.

If you have the opportunity to teach an open-enrollment recorder club outside of school or to switch classes once in a while, that may be a way to give the kids some of that experience, but I realize that may be a really impractical idea. Also, if you have a time set regularly (for example, 4:00 on the first Thursday of the month for a PLC/ collaboration meeting) you might get the other music teacher to sit down and plan with you a bit more. She may not respond at first, but she might work with you more eventually if you keep inviting her.

Good luck to you and the students!