The Degrassi Franchise has been retconned three times. The first was by a year for Degrassi Junior High. This was then followed by a second retcon for Season 10 by two years, then (recently) a third retcon for Season 13 by one year.
Retroactive continuity (often shortened to retcon) refers to the deliberate alteration of previously established facts in a work of serial fiction. Retcons may be carried out for a variety of reasons, such as to accommodate sequels or further derivative works in the same series, reintroduce popular characters resolve chronological issues, reboot a familiar series for modern audiences, or simplify an excessively complex continuity structure.
Degrassi Junior High
Although many first-season Grade 8 characters have birthdays in 1974, since Emma Nelson, daughter of Spike, was born in June 1989, and since Grade 8's start at the age of 12 or 13 and finish at 13 or 14, 1974 birthdays should be retconned as 1975. This turns the Class of 1992 into the Class of 1993 and the Class of 1993 into the Class of 1994.
Degrassi (Season 10)
Although Degrassi: The Next Generation Season 8 and 9 depicted the 2007 - 2008 school year, beginning with Degrassi (Season 10), the series has been retconned to the current year. It depicted the 2010-11 school year.
Degrassi (Season 13)
Although seasons 10-11 depicted the 2010-11 school year, with seasons 11-12 depicted the 2011-12 school year, in the finale of Season 12, the series was retconned to the current year. This turned the Class of 2012 into the Class of 2013, and so on. Consequently, Season 13 depicts the 2013-14 school year.