I live in Oregon and it sounds like a 504 plan is called an IEP, Individual Education Plan in Oregon. The IEP is written by the professionals who have evaluated the child and then the parents are asked for input and approval. Your school should be able to help you get the process started.
For my grandson and granddaughter, the first step, after their pediatrician recommended further treatment, was an evaluation by the Intermediate Education Service District which is part of the local school district. There is no charge for the evaluation. When the evaluation shows that the child needs special treatment to be successful in school, a team of professionals provides treatment and works together to write the plan.This is a part of the President's "No Child Left Behind" legislation. I found quite a bit of information about the IEP and what it should include by googling IEP.
Other terms that are helpful for find information on the Internet are special needs and education.
I am sorry to hear that Sandy S. has had difficulty at her son's school. My grandson is in a special education class because of a speech disorder, possible sensory processing disorder, and some behavioral difficulties. The program has been of great help. My daughter, their mother, receives help from all of the people involved in making sure that her children are getting the help that they need. They regularly communicate with her.
My granddaughter is in a regular classroom and has received some extra tutoring in math and reading because of having ADHD. Her school has a full time counselor who has provided help in making her IEP a success.
I have had much personal experience with bi-polar disorder. I'm surprised you haven't needed a plan for his education. You don't say how old he is. Perhaps he's had especially sensitive teachers until now. But there could be other reasons that he needs a more directed educational plan that are unrelated to the teacher. My first thought is that the teacher may not understand the dynamics of the illness. My second thought is that he is now older and his classmates are acting out more that they were when they were younger. My third thought is that being older may also mean that he's handling school differently than he has in the past.
I would hesitate to blame the teacher. My first approach would be to assume that she's not knowing how to handle this situation. The 504 plan, if I understand what it is, should do that. Making an educational plan has to include professionals as well as parents. I would start by asking the principal of your son's school who to call to get help writing this plan. I would then work with the office that organizes special education in your district.
When I use the term special education I'm using it in the new expanded version. It does not necessarily mean a special education class separate from the regular classroom. It means providing children with the resources they need to succeed in school. Legislation has provided more structure so that children can get the type of help that they need. The evaluation and educational plan is provided by the school district at no cost to the parents.