Where in the novella is Dickens suggesting that Scrooge’s childhood was unhappy?

In stave two of A Christmas Carol, Scrooge is visited by the first of the three spirits, the Ghost of Christmas Past. The Ghost shows Scrooge a number of episodes from his past, including his school days. Scrooge was deeply unhappy at school; he was a very lonely boy, whose friends went home for Christmas while he was left to spend the holiday season all alone. But in the vision that the Ghost shows him, Scrooge is absolutely delighted to see his beloved sister Fen arrive at the school to take him home for the holidays. Their father is much kinder now, she says, and has consented for Ebenezer to come home.
Our little glimpse into Ebenezer's school days is very important for two reasons. First of all, Scrooge's isolation and loneliness as a child parallel his current situation as a grasping, ornery old miser who hates Christmas. Secondly, we can see that it wasn't always like this; there was a time when Christmas brought great happiness to Scrooge. This foreshadows Scrooge's redemption, when he will rediscover the joys of Christmas that he experienced in his younger days.