I had the same idea as Peachey at first, thinking that it has to do with the use of sheep as people who follow blindly. But then I did a little research and found this idiom: separate the sheep from the goats. To be honest I've never heard it used before. It originates from the Bible and means dividing people or things into superior and inferior groups. Here's a site about it: https://idioms.thefreedictionary.com/separate+the+sheep+from+the+goats There's even a sample sentence about students: When you choose the students for your homeroom, don't separate the sheep from the goats and give me all the troublemakers! Maybe your book is referring to this idiom?

Without any more context, here's what I think you want to know: sheep = people who follow directions willingly, and without question (there's a word: "sheeple", ie. sheep+people, which means a group of people who don't question anything.) goats = people who decide their own lives.