I went with a shorter version that included a lot of the editing suggestions.
The Principal replied to my email that same evening and asked me to call the transportation director the next day. He also said he'd call them as well because he shared my concerns. Unfortunately school (and school offices) were canceled due to snow so I wasn't able to call. I had planned to call yesterday but I received another email from him very early, saying he had forwarded my email to the director and had spoken to her, so I decided to leave it at that. If there are any further issues I'll call her but I think it's covered for now.
Yesterday the boys told me they had yet another substitute driver, and she again needed a bit of guidance, but she was really nice and knew most of the route. DS9 told me that right before he got off the bus he stopped and said to her "You're a lot nicer than the lady we had before. She yelled a lot!" I asked him what her response was and he said "She didn't really say anything, just smiled at me." I can imagine.