hubaeneun donggideuri chucheonhan hagwone danigiro haesseo.

후배 means a junior—someone who is younger or lower in rank/experience than the speaker or another reference person in a school, workplace, club, etc.

It does not simply mean younger person in general. It is a relationship word, usually opposed to 선배 (senior).

So in this sentence, 후배는 means as for the junior... or the junior... depending on context.

동기 refers to people who started at the same time in the same school, company, program, military unit, and so on. A common English translation is peers, classmates of the same cohort, or people from the same intake/class.

Adding 들 makes it plural:

• 동기 = a same-year peer / cohort-mate
• 동기들 = same-year peers / cohort-mates

So 동기들이 means the peers who started at the same time are the ones doing the recommending.

No, 들 is often optional in Korean when plurality is already clear from context.

So both of these can work:

• 동기가 추천한 학원
• 동기들이 추천한 학원

Using 들 makes the plural idea more explicit: it tells you there were multiple peers recommending the academy.