When cancelling an ED extension, how long before the extension or school letter end date can you cancel?

I’ve got the school letter saying the official course end date is 5 days after the extension ends but was planning on leaving two weeks before the extension ends.

Ed-Visa question : I'm on third yearly extension of stay studying a Bachelors degree. My extension of stay is valid until dec17 and the official end of my last term will be dec7. BUT I'll be done with all courses and really everything for my degree by the end of august. University never had a foreign student before doing an entire degree with them (only some exchange students from china and Cambodia) and zey don't seem to know anything about having to call in with immigration and cancel my extension of stay. They said I can stay until the end of my current stamp. What do you think, if I don't have any more proof of actually studying from August onwards, but technically still am a student until dec7, what should I actually be doing about it. Any thoughts? Thanks y'all, you're always great!