This is the 14585th most frequent Russian word.
первоклассника
A first-grader.
Here, 'первоклассника' is in the singular masculine genitive case, showing possession linked to 'портфель'.
Портфель первоклассника лежал на столе.
The first-grader's backpack was lying on the table.
In this sentence, 'первоклассника'—again in the accusative case—is used to show the direct object of 'позвал'.
Учитель позвал первоклассника к доске.
The teacher called the first-grader to the blackboard.
The word 'первоклассника' is the singular masculine accusative form of 'первоклассник', a noun meaning 'first-grader'.