This is the 14051st most frequent Russian word.
одноклассников
"Classmates" (plural form).
The word 'одноклассники' is used in its plural genitive form 'одноклассников', meaning 'of classmates', as the direct object of the verb 'встретил' (met).
Вчера я встретил некоторых своих одноклассников в парке.
Yesterday, I met some of my classmates in the park.
The word 'одноклассники' is again in its plural genitive form 'одноклассников', used to denote the group receiving congratulations in the sentence (object of a verb in passive).
Одноклассников поздравили с окончанием учебного года.
The classmates were congratulated on the end of the school year.
Here, 'одноклассники' is used in the plural genitive form 'одноклассников' to show possession, as in 'of our classmates' (object of 'вспоминаем', recall).