I assume that the School marked students on the basis of 1-5 in each subject and 3 was the pass mark.
The Russian Wikipedia article on school grades says:
Традиция обозначать цифрами прилежание и успехи учеников утвердилась в России ещё в начале XIX века. Тогда в гимназиях употреблялись цифры от 0 до 5. Нуль показывал, что ученик совсем не исполнил своих обязанностей; если он получал два нуля подряд, то он подвергался телесному наказанию (до 1864 г.)[1] Единицу и двойку ставили тогда, когда ученик неудовлетворительно приготовил урок; тройку ставили за посредственное прилежание; четыре — когда ученик хорошо исполнил свои обязанности; пять он получал только за отличное знание урока. Учитель был обязан ставить баллы в классе, характеризуя только знание заданного на дом урока, и не имел права учитывать внимание или рассеянность учеников во время занятия, а также временное или постоянное прилежание ученика, его возраст и способности[2].
Несмотря на многочисленные попытки реформирования этой системы, из которых наиболее значительная была предпринята при Луначарском[3], эта система сохранилась и в СССР.
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"Traditionally the numerals denoting the diligence and success of students were established in Russia in the early XIX century . Then, in the gymnasiums were used numbers from 0 to 5. Zero showed that the student had not fulfilled their obligations; If he gets two zeros in a row, he was subjected to corporal punishment (up to 1864), one and two were given when the student had prepared a lesson unsatisfactory; three indicated mediocre diligence; four - when the student has well performed his duties; five he received only when displaying excellent knowledge of the lesson. The teacher was obliged to give the points in class, describing only the knowledge of a given home lesson, and had no right to take into account the attention or distraction of students during classes, as well as temporary or permanent diligence student's age and ability.
Despite numerous attempts to reform the system, of which the most significant was made by Lunacharsky, the system continued in the Soviet Union."
The article has a link to the concept Двоечник - 2er / dunce, who had to retake a year before passing, so it fits with Ann's assessment of 3 being the passing grade. (From a modern Norwegian viewpoint, with six pasing grades, I must say that just three passing grades was rather limited, probably an indication not only of the strict and high standards of the day, but also of the narrow gauge of assessment, i.e. there was a limited syllabus and things were either right or wrong.)
