I just wanted to say I am so sorry to hear that Holly is having to go through this already. My heart breaks for her. Mockingbird has a great idea. Something, anything, to try and make this easier for Holly. Perhaps the teacher would consider contacting the other parents in the class and suggesting this project to help Holly get through this tough time. Does she only need to wear the mask in class or does she need to put it on as soon as she gets out of the car? Does she need to wear it anywhere else? If the teacher and parents (cause I'm sure the parents would have to) agree, perhaps Holly wouldn't have to put her mask on until she gets into the classroom, and they could surprise her when she walks into class and gets ready to put her mask on.
At 5, I'm guessing she's in kindergarten. If for some reason Mockingbirds idea doesn't work out perhaps you can withdraw her for a month, teach her at home, and then put her back in school. If the school wants to they CAN work with you on that. My aunt did it for my cousin when her husband got sent to Norway to work for a month. My cousins teacher gave her all sorts of work to stay caught up with math and stuff like that then had her keep a journal and report all the stuff she learned covering history and social studies. She actually had a conference call with her class once a week. When she got back she re-enrolled in school and there were technically no absences. She didn't recieve any grades because she wasn't actually enrolled in school but her teacher did review her work to make she she wasn't behind when she returned. So where there's cooperation, it can work.
I hope this all works out for Holly. What a horrible way to have to start out your school years.