I'm not a school teacher, but I am a school nurse and have been in meetings such as this, due to the students diagnosis. This makes me so mad!!! No teacher should talk to a parent with that tone. She showed you no respect. Take this to the school board and demand respect. I am so sorry you had to go through this and more sorry your son had to witness her non professional attitude. You have every right to fight for the right of your son. If you feel he should be in this math class and he wants to continue it, then fight for him.
There are also parent advocants (sp) that will help you. Research and find someone to help fight for his educational rights.
Tenure means she has over five years in that school district and it will take a act of congress to fire her, but they could put her in a different classroom or make her teach something other than math. I'm sure your not wanting to get her fired, but demanding the respect your son deserves.
Teachers like her shouldn't be teaching. Sounds like she has a problem with your son, which is wrong.
Good luck!!!