If the sentence is exactly as you've represented to be, the teacher is spouting nonsense . *"Was she and her sister at the zoo?" is a yes/no question with subject-verb inversion. Turn the question into declarative and it's obvious that "was" is incorrect:
*She and her sister was at the zoo.
*She and her sister was not at the zoo.
These two are ungrammatical, but
She and her sister were at the zoo.
She and her sister were not at the zoo.
these two are grammatical. If the teacher doesn't agree, then the teacher doesn't know enough grammar to be teaching English to anyone. Tell the principal.
3nafish's answer makes some good points about technicalities that may be involved in the teacher's answer, however, so be certain that those weirdly punctuated versions and that final syntactically weird version of this sentence aren't what the teacher was talking about. Not that there's anything wrong with or ungrammatical about 3nafish's examples, mind you. They just aren't the easiest and clearest way of expressing the content of the sentence you quoted -- unless, of course, they are appropriate for the context in which the sentence appears.