Complex sentence: Since the 2008--2009 school year, new first-graders are using the 12-year system, and a full transition to the 12-year system will not occur until the 2019--2020 school year, when the current third-graders graduate.
Split into simpler sentences: Since the 2008--2009 school year, new first graders are using the 12 year system. As such, full transition to the 12-year system will not happen until the 2019--2020 school year, when the current third graders graduate.