Primary and secondary education is like a production line of shoes where each shoe is passed on from one factory employee to another until the shoe is completely assembled. The mission of the factory at the end of each day is to produce finish products in the right quantity, right time, best quality, and least rejects.
Adding another year is just like adding another factory worker in the production line. It will not increase the quality, but only slow down the production process. If you want to increase quality, pay attention to quality of school environment, quality and training of teaching staff, and technology that you use in teaching, BECAUSE these three are not yet optimized, there is still a lot of inefficiencies in these aspects and plenty of room for improvement.