Walking pneumonia can be caused by different things, but often, it is by the Mycoplasma bacteria.
My daughter got it one summer, in July, shortly after we moved to Texas. A few weeks later, I ended up with it, just about the time school started. My daughter weathered it really well, but it really knocked me down.. it didn't help that we had 4 little ones, including an 8 month old.
I finished the medicine on a Sunday, but was still coughing (as I knew I would). I had a return doctor visit on Friday, but by Wednesday, I was coughing so bad I went back to the doctor..... I ended up with a really bad case of pneumonia that seriously, almost landed me in the hospital... I went to the doctor every week for a month just so he could make sure I was getting better. Truthfully, I think it was at least 2 months before I really had my strength back.
Why did it hit me so hard? I think I ended up with what the doctor called "Bacterial Rebound" ... the antibiotic also killed off the good bacteria that kept the bad stuff in check, and that blossomed.... I had been under a lot of stress that year.. (new baby in November, my father passed away in December, hubby getting transferred, two trips for house-hunting, and the move itself, all in about 9 months...)
Anyway, I don't really know what to suggest... most kids do rebound off things like that pretty quickly.
What is it she is going to miss? Could you take her just for that, and then pull her out of school?
I would try to make sure she is getting plenty of rest for a while....