I assume that this sentence can only be addressed to a group of people? For a single friend/child (informal) you would say vai allo zoo or the polite form vada allo zoo?
It's quite muddling because the 2nd person plural is also andate so the sentence ending with a . means "You go to the zoo", with a ? means "Do you go to the zoo" and with a ! means "Go to the zoo". Of course in reality context would settle it.