To fire a teacher you need documentation. This is a teaching phrase that refers to a specific process that explains why a teacher is bad.
To create your own documentation you need a notebook of paper or something similar (3-ring binder). Give it a title and all that, then record the first incident of bad teaching.
Include the date and a description of the incident, include all those involved. It doesn’t have to be more than a few sentences.
Continue recording these incidents over a few months, maybe even ask others to help you.
If this seems like a lot of work you can ask a principal or school administrator to do it for you. They should already have a file on every teacher and be able to follow the district policy for documentation.
You may run into some difficulty here if you sound more like a bad parent than a good one. Remember that principal and administrators time is overwhelmingly spent on problem students. These are often nasty situations involving divorce, social services, and, of course, bad grades, bad behavior, and detention.
You have to stand apart from that and saying something like “last year we never had a problem” doesn’t work. New skills are required in each grade level and, all too often, students struggle in a higher grade and parents blame teachers. As if fractions weren’t hard enough, students have parents who don’t help them with fractions and instead blame teachers.