At least this 1984 report on "Aspects of Soviet Secondary Education: School Performance and Teacher Accountability" https://www.wilsoncenter.org/sites/default/files/media/docum... claims that a more ambitious curriculum did not lead to more repeaters ("grade repeating has declined over the years"), but instead "most teachers have chosen the easier solution of adapting their grading practices to the results expected of them." The students who didn't get algebra were apparently filtered out later: "According to the few statistics available on entrance exam results, it seems that somewhere between 20 and 30% of all students fail tests in their chosen field of specialization."