"What he would write if he could, if it were not for Mr Whelan reading it, would be something darker, something that, once it began to flow from his pen, would spread across the page out of control, like spilt ink. Like spilt ink, like shadows racing across the face of still water, like lightning crackling across the sky."
As in the quotation in my second post about Boyhood, Coetzee here seems to be making an indirect comment on his own writing. I found several citations of this passage on the web; interestingly, all of them omit the phrase "if it were not for Mr Whelan reading it." Here, Coetzee is remembering his boring and frustrating school in Cape Town (not the school in Worcester that had been more fascinating, even if the fascination was a matter not of the teaching and learning but of an education in cruelty and pain) and the teacher of his English classes.
By taking out the reference to the teacher, those who cite this passage can more easily read it as an allusion to Coetzee's later authorly goals. But the reference to the teacher highlights the role of the expected audience in writing: the young John's knowledge of his audience prevented him from being able to write what he wanted to write. It is not a creative ambition that is at issue here but the self-censorship that arises when an author is aware of the limited range of expectations that a specific reader (or readers) may have. It almost reads like a warning to would-be writers: don't write for a specific audience; don't let the limitations of your audience limit you.