I hate the "just another headache" mentality. I am so sorry you have had to navigate this.
My son went back to FtF school last year, and we told the teachers, the office staff, and the principal he gets migraines. We told the he often has other side effects, and if he says he has a headache, please call us. We'll come and get him.
We told him to tell them if he feels one coming on, he needs to ask his teacher or the office staff to call us so we can bring him home.
During his first week back he got a migraine. He told the teacher (who was a sub), he had a migraine and that his get really bad and he might throw up. The sub told him, "everyone gets headaches. You'll be okay."
What happened? He threw up. He was so embarrassed.
They called us. He came home.