Scrooge was transported to his childhood school.
When the Ghost of Christmas Present first took him back, Scrooge was taken back to a vision of himself as a child. He remembered it as if it were yesterday. He enjoyed the vision very much at first, because he saw his old schoolmates and was pleased to see them.
Then he saw a vision of his former self, a young Scrooge, all alone in the schoolhouse as it was deserted when everyone else went home. He became quite a bit sadder.
“The school is not quite deserted,” said the Ghost. “A solitary child, neglected by his friends, is left there still.”
Scrooge said he knew it. And he sobbed. (Stave 2)
The reader might be surprised to see some emotion coming out of what before appeared to be an unfeeling Scrooge. However, Scrooge reacted with sympathy to Marley, and reacted with surprise and glee when he saw the boys. The shell that Scrooge was around him is beginning to soften.
The visions that the reader sees of a younger Scrooge begin to explain why he is the man he is. A little boy, all alone in a dreary schoolhouse? What a sad situation.
[They saw] a long, bare, melancholy room, made barer still by lines of plain deal forms and desks. At one of these a lonely boy was reading near a feeble fire; and Scrooge sat down upon a form, and wept to see his poor forgotten self as he had used to be. (Stave 2)
These visions serve two purposes. Scrooge is forced to remember how his father used to leave him at school for the holidays, where he had nothing but his books to keep him company. The reader learns what made him the lonely curmudgeon that he is, and starts to feel some sympathy for him.
Throughout the story, Dickens weaves a picture of Scrooge through the past, the present, and the future. He will have to face what he was, what he has become, and what might become of him before he can change. In this scene, we see Scrooge just beginning to crack.