IMO in The Voyage of the Dawn Treader Lewis is trying, simultaneously, to work out his own bad feelings about his experiences in traditional (horrible, Dickensian) boarding schools and also mock progressive trends in education that he dislikes and these two things meld into something very odd.

I also once saw someone respond to the book's first line ("There once was a boy named Eustace Clarence Scrubb, and he almost deserved it") with "Bold words from a man named Clive Staples Lewis," and ever since then I've had a sneaking suspicion that Eustace is a little bit a self-insert.