Clever as you boys have been, I'm afraid it would be quite irresponsible of me to return so mischievous a tool to the hands of vagrants such as yourselves. I am quite certain that should you gain access to such a document once again, you might have a leg up, to coin a phrase, upon the new administration.
And that simply would not do.
Best of luck with your future endeavours.
This note is written with easily siphoned ink on a bit of parchment that looks suspiciously like something that may or may not have been pilfered from Filch's desk. The marauders are sure to recognise their map