I think the school bus driver could have handle the situation better. Going back to the house a pulling the the girl and her sister on board the empty bus was wrong. That would make alot of parents mad. Also, parochial schools are not jam packed with intolerant bigots - I know because I went to parochial schools for 12 years.
The school bus driver is going to have to realize that being tolerant means dealing with other people's opinions that are different from hers, it also means she's gonna need to learn how to have appropriate dialogs with minors. It's a controversial topic, student is a minor, school bus driver is an adult, you got to be careful how you interact when feelings and emotions are raw. This is a law suit waiting to happen.