Why can you not go back to high school??

Question

I am in a situation where I missed 4 years of High school. I then sat my GCSEs and got G grades as a consequence (apart from a D grade in Englaish) I wondred I know this might sound like a strange question.....but why is it that once you reach 16 /or are in a situation like mine that you cannot go back to high school.(re-do the years again) Why is this.?
Is it against the law?

Answer 1

Because most places only provide free schooling to a certain age. It may be possible to retake the missing years at a private high school, but generally, in the interests of time, an equivalent diploma is given via examination.