In most schools in Russia, there is more than a roomful of students in each grade, so, e.g. the 5th grade is subdivided into 5a, 5b, etc., and
"определение детей в классы внутри параллели" is "allocating students to different classrooms within the given grade".
In some schools, this apparently occurs at random, whereas sometimes, for example, stronger students are grouped into one room, and weaker ones into another (this was the case in my school).