What to do about my migraines and the school?!

i'm a senior in high school, so missing class isn't much of an option. But i get very frequent migraines (at least once a week.) and sometimes (not always at least) have had no choice but to stay home as i'm prone to throw up, and am very sensitive to light and sound. I've been to the doctor, had scans, blood tests, etc. done and they can't find anything. But they wrote a note to school saying i was to stay home if i have a migraine to keep on file (the school requested proof i really had migraines..). Well, even if my dad writes a note or even calls the school saying i'm staying home due to one, they won't except it and still count my absence unexcused unless i get a doctors note for EVERY time i have one and miss school. We simply don't have the money to go to the doctor so they can tell me to take some aspirin and go home and take a nap. I'm so frustrated, what should i do?

Sorry so long!
thanks.
xxx