You have to be firm with her, and not try to just wait it out. If she misses too much school, she will never go back.
Still, you need to realize that her feelings are real. Showing anger won't help either of you.
She may have one of three conditions that you need to determine: school phobia (fear of going to school), separation anxiety (fear of leaving you or your home, or agoraphobia (fear of crowded and public places).
Talk with her teachers. She might be having trouble on the way to school such as bullying, embarrassment, or abused. These things could also be happening once she gets to school. Identify the problems so that you can work with her on them.
Take her to the doctor for a complete physical examination. Tell the doctor the whole story and ask him to rule out any serious illnesses.
If he rules out an illness, then believe what he says. Don't have a lot of expensive tests. Assume that your child is physically well and needs to go to school. Keep assuring her firmly and confidently that she'll be fine (and so will you) once she arrives. If she still claims of physical ailments, you have two options;
The first is that you insist that she go to school unless there is clear, measurable, evidence that she is sick, for example having a temperature, obvious diarrhea and vomiting, etc. Just "feeling unwell" is not enough to miss school, after all, many adults have to go to work with headaches or other symptoms.
Option two is to believe what she says. If she claims to be too sick to attend school, then she is too sick to be up at home. Make sure she gets into bed. Turn off the lights, close the curtains, don't let her watch TV and don't bring her any snacks. Just maintain your daily routine. Make staying home a complete bore. If she isn't asleep then she could be doing her homework. She definitely shouldn't have friends visit.
Establish some incentives for her to go to school. Some kind of reward or privilege would be in order.
Be firm and remain calm. Let her know that you expect her to go to school, but don't argue with her if she resists. The goal her is for her to want to go back to school. Once she goes and finds out that she's fine, her previous symptoms should disappear.
If you have no luck with these techniques and fear that she may be seriously depressed or anxious, then call your doctor for a professional referral.