Do you have a written plan for her medical issues with the school. If not you need an <a target=_blank class=ftalternatingbarlinklarge href="http://en.wikipedia.org/wiki/Individualized_Education_Program">IEP</a> desperately.
You can't keep her from getting sick, but there are several things you can force them to do:
*Allow enzymes, extra drinks-including in class, free restroom privledges if needed
*Allowances for absences; including providing a school-paid tutor/teacher at your house if she is out for more than X days.
You don't have to homeschool for her to get a good education and not be penalized for having CF.
In addition, have you talked to her teacher & the school nurse. They should understand the issues and the standard precautions such as hand sanitizer in the classroom that could improve her environment.