I think you cheated a little in your cut-and-paste. `student_by_school` is not defined in the code you've shown. What you *did* define, `student_school_list` doesn't give you what you want if you use `defaultdict(list,student_school_list)`.
I thought for a moment I might just use:
[(b,a) for a,b in student_school_list]
--
Keeping medicines from the bloodstreams of the sick; food
from the bellies of the hungry; books from the hands of the
uneducated; technology from the underdeveloped; and putting
advocates of freedom in prisons. Intellectual property is
to the 21st century what the slave trade was to the 16th.