Is the number of absences due to migraines a significant number?
Some colleges really won’t see such a student as a good fit.
Even less selective colleges may not have any accommodation or alternative path to allow a significant number of days of absence. Making up labs or contributing to group projects is difficult. Attendance still matters for many majors.
Yes, having had 4 students in college at certain points if time with Covid, and reading college parent Facebook pages, I’ve seen cases where professors have been pretty tough on students who were sick/injured. It’s very different from HS. My niece who gets migraines struggled a lot, anxiety too. Her dad had to call her every morning and stay on the phone until she was on her way to class, but she did graduate. Some have no attendance policies, some have very strict ones with no exceptions. Some students are told to take the W.
She will be prepared as this is what she’s done all throughout HS. Despite having a disability, she never asked for anything. If she missed something, she taught it to herself or arranged with teachers how to make it up with no oversight from me.
If she tells other students what the penalty for staying home due to contagious sickness is, then the school should expect more students coming in sick and spreading things like flu, COVID-19, etc.
Not terribly significant. An average of 3-4 a month. Fortunately she may be turning a corner with these as she started taking nutritional supplements and hasn’t had any for the past 2 months. 
This disability will need to be on record with her college. And even with that, some still have a strict attendance policy. So please do check once her acceptances start coming in.
Good luck to her.
Missing so much class is difficult as the semester goes on and later assignments and exams weigh more.
I feel like large colleges may have telework asynchronous infrastructure or programs or ADA accommodations, but some are understaffed leading to delays.
Depending on the major and faculty, small colleges may not have flexibility either. There is concern over cheating and duplicating the workload of faculty.
It is not easy to discern which college may be more helpful for your dear daughter.
You should line all your ducks request a 504 for the second semester of school… Ask for extended time on quizzes, tests and assignments, quiet , separate location (if she needs this ). you should also be prepared to have. 504 plan registered with the college office of assessibility services.
The private school may be exempt from 504 law.
Edited to add article by a lawyer in CA.
Edited a second time… there may not be enough time to get recognized as disabled by migraines for hs. Will be needed for a chance for accommodations in college, though.
If the hs took federal pandemic $, maybe some benefit may be seen this spring.
Where has OP’s daughter applied besides the UC system? Most private schools have connections with private colleges they could utilize on behalf of the student to explain the problem.
They should show the 504 but it’s up to the college what accommodations they provide.
The high school, if private and accepts zero federal funds, is not obligated to implement a 504 accommodation plan…I believe. But the parent can check this by calling the CA Protection and Advocacy Office there.
Re: colleges…the student needs to work with the disability office.
This isn’t the issue. The issue is…too many absences.
A standardized test score would be useful in this context
Like what?
It would depend upon what the C grades are in. For example, if in math, that would be offset if an SAT score of 750 in math were presented.
A problem is that AOs have no basis to judge performance other than the low grades, so if there is any countervailing data, she should show it. No one is going to accept a bald assertion that she would have received an A but for attendance, unless it comes from the teacher or counselor.
It is good the OP vents here.
It is good the DD loves her high school.
She for sure is learning life lessons more important than admission to highly selective colleges.
She will have alumni network with the hs no matter what college she ends up at.
Thank you for the clarification. That is not what you indicated in your initial post.
(I teach nursing. Students would likely be asked to withdraw if they miss more than 2 lab/simulation/or clinical days during the semester.)
I believe the difference between the “athletes” and OP’s child is that the athletes may very likely set up alternatives before the test. I know of teachers who make students take their tests before they miss school. Randomly asking to take a test after everyone else truly does increase chances of cheating for the person taking it after everyone else.
I urge you to not do this while you have children in the school.
You are right. She’s my last one and I will wait till after graduation. By then I should be thinking more clearly. No doubt though, I will give this school a one star review in as many online sites as I can!