Adventures of Huckleberry Finn
Judge Thatcher, Miss Watson, The Widow, Aunt Sally, and All Those Civilized Folks—Collectively
Yes, this is a group mentor. As Huck darts back and forth between the wilderness and the "civilized" world, he gets guidance, both directly or indirectly, from the adults that really care about him. (Needless to say, this doesn't include Pap, the duke and the king, or that random drunk guy.) They teach him manners and religion and all that, but of course the point is that Huck doesn't just accept these teachings—he challenges them.