One of the things that we can't necessarily know is whether Terry Pratchett was likely to become Terry Pratchett, given his beginning. We never do hear about the failures. It's not just that mediocre students often fail to make something of themselves- how often do mediocre students with a mind like Pratchett's accomplish world changing things like Discworld? If you knew a priori that you had such a mind, and wanted to maximize your contributions, would it be a good idea to be an aimless student?

That said, it may be that you wouldn't be likely to get a Pratchett without a certain baseline of failed attempts; and that a youth spent in such a way may be bad for the individual on average and good for society in aggregate. It would be an interesting dilemma.