учеников

In a sentence

• Учитель отметил достижения своих учеников.

  The teacher noted the achievements of his students.

  Here, 'учеников' (students) is the plural form in the genitive case used to indicate possession, referring to the students being taught by the teacher.

• На уроке присутствовали десять учеников.

  Ten students attended the lesson.

  In this case, 'учеников' (students) is in the plural genitive form used after numerals to quantify entities.

• Все ученики школы участвовали в соревновании.

  All students of the school participated in the competition.

  Here, 'ученики' (students) is used in the nominative plural form as the subject of the sentence indicating who is participating.