"동급생은 동반자에게 공부하자고 했다."
Translation:The classmate said let's study to their companion.
The quote "Let's study" should be accepted either in the middle or at the end of the sentence.
"Their" is not in the Korean sentence and can therefore only be assumed. Answer without "their" should be accepted.
This Korean sentence is very strange. Koreans never say "동반자" in everyday life. You can encounter the word only in old songs.