Thanks @Naomi. I learn something new every day. :-D
@Theland,
Regarding "mitigating reasons for the error"; I hope these do not backfire - for example if your daughter had been subject to lack of sleep due to neighbourhood noise or her home/family situation had been causing stress issues, poor concentration etcetera. They may try to turn this against her, in that they might have procedures in place where she was supposed to notify them that her performance was impaired _on that day_, enabling them to shift her to other duties and bring in a substitute with the same qualifications.
Having said that, I can't think of many jobs where it is considered normal to dismiss someone over a single mistake, even when it is a public safety issue.