Ms. Frizzle took the children on a dangerous field trip to Pluto upon which a student perished, and she abandoned the kids unsupervised. Because she was in a unionized public school district, she wasn’t fired. She continued to push the envelope, even taking students inside another students body, but no disciplinary action was taken. At a non-unionized charter school, this wouldn’t happen.
As a progressive liberal, I think protecting our kids is important. And lazy teachers have unions to protect them from both doing the bare minimum AND putting students in a position to kill themselves in outer space. You know she’s not a good teacher because pluto isn’t even a planet!
Mr.Frizzle is functionally a timelord with a TARDIS. She wouldn't have much problem finding a replacement Arnold.