I would also like to add for context that, setting aside the hows and whys of whatever happened, the fact of Kayla being dropped off at school alone, without anyone there who has been instructed to pick up Kayla and escort her, is more than likely (in my experience in SPED) out of compliance with what would be outlined in her IEP (Individualized Education Plan) for her transportation services. The IEP is a legally binding document and activity that is out of compliance with what is mandated in the IEP is illegal. BTW, I am not saying that anyone should be fired or sued. I am simply giving my 2 cents as someone who has a lot of experience with IEPs and is almost finished earning a master's in special education.
If something, such as this terrible incident, happens as a result of being out of compliance with IEPs, the door opens to lawsuits and a vast array of other legal issues. And that is for cases far, far more minor than this one. I pray Kayla is safe and warm, and can be safely recovered - Even so, I don't think that will be the last of this. This was a big mistake. The district is screwed IMO.
Transportation for kids in special education, in most cases, is noted in the IEP under what is called the "related services" (that the student receives, provided free of cost by the school district). In Kayla's case, I am 100% confident that it would have been, seeing as she had Down's syndrome and received specialized transportation services (but again - this is just my opinion as a special education professional who has experience with IEPs and almost has a master's - take it with a grain of salt if you would like). The transportation services on the IEP are very important because, as with the rest of the IEP, it is legally binding.