My personal take on this aspect of canon was that this meant there had been 22 educational decrees since the dawn of time... Or since the institution of some 'enlightened' reform of the decree-accounting system back in the 18th century... Or since the Ministry bothered to meddle at all with what the teachers get up to in that school... Or something like that. It might have been a decade or a century since E.D. 21 came down, or there might have been 21 decrees issued when the school was first chartered and nothing since. ;)