I'm not real sure that, at 48, you'd get me out to school/work in a mask!
I really feel for her. I agree with many of the Moms here who are advocating keeping Holly at home a while until she's stronger. Look into some of the different immunity preps that are available on top of good, diligent treatments. More than anyting, your daughter needs to not be afraid of learning - it is a lifetime ambition and blessing that we are able to have an education nearly at our fingertips.
At 5 you can worry more about socializing her later. Look into homeschool groups or day care groups in your area and see if you can form a small circle of friends her age than may (or may not) be from her school and work with their mothers/fathers to have social times for Holly. Find some who are compasionate towards the health issues you face and are willing to encourage their child to go the extra mile and help Holly stay healthy. Let those kids see into Holly's daily routine and teach appreciation for our differences.
I think it's great that the teachers are willing to make changes in their classroom to encourage everyone's health (if they are singling it out as 'for Holly' that could turn ugly in a couple of years when kids get meaner). You mention that the other class has a CFer in it to. Can you talk to that parent and see if they are experiencing an increase in colds and yuckies as well? Remember that this is such a varied disease and we are all affected at different levels throughout our life.
I agree that keeping Holly home is, in this case, far better than dragging her kicking and screaming into school. What aobut 1 or 2 days a week? If you also work outside the home do you have help to watch her when you are at work? If not, can the school come up with a way for her to go but have limited exposure to germs?
I wish you well.