Hi Mark:
I can so sympathize with seeing our children in pain and not knowing how to help them. My sympathies are with you on that!
There are SO many reasons for headaches on a continual basis. Eyestrain, posture, neck and shoulder problems, jaw tension, right on down to allergies...especially, food allergies!
If this were my daughter, I'd start with the most obvious things to have checked out such as eyestrain (opthamologist/optometrist), jaw clenching/TMJ (Dentist), neck and shoulder problems (a good chiropractor), and I'd also ask my family doc for a referral to an allergist to check for food/airborne allergies! Food allergies certainly cause headaches and it's usually something that is eaten in everyday diets such as wheat or food preservatives. An "Elimination Diet" may do the trick!
I have a friend whose daughter was in constant pain everyday with headaches and missed a year of school. After seeing several neurologists (should also be checked with as well), they couldn't find any reason for these nasty things. In desperation, my friend took her daughter to an allergist who put this young lady on a diet of least allergenic foods for a number of weeks (quite boring for the child to have to stick to but worth it!). After a number of weeks, one food at a time was re-introduced to see if the headaches returned (they had disappeared completely after a few weeks of this diet). When a food caused a headache again, it was left by the wayside and not consumed for a period of time. They did this with foods until they had a list of the "offending foods" and eventually, this young lady was headache free!
My very best of wishes to you as I know that this is heart wrenching. Please keep us updated.
Did you go to a neurologist? Any more symptoms?