I concur with the other posters who have suggested making all communications in writing and getting the higher-ups involved. While not with grades, we had issues with an IEP and BIP- teachers were not implementing but complaining about my son’s poor behavior at the same time. My suggestion that they follow the BIP (that was PROVEN to help my son) fell on deaf ears.
I started putting everything in an email. And not just to the Special Education teacher, but also to the Principal, Vice Principal, and every General Education teacher he had. When that didn’t work, I looped in the Director of Special Education for the district. I made sure I knew the process, so I could say things like “if progress is not made by [date], I will be proceeding with filing a formal complaint with so-in-so”. We got a meeting and when I showed up for it the front desk receptionist at the school told me that she had never seen the Director of Special Education EVER come to a meeting.
You can be nice while also being firm and not letting yourself be stepped on.