Return to your seats when you're done talking to the person you are talking to now
Go back to your seats after you finish talking to the person you are talking to now
Both of these statements work equally well and are both grammatically and logically correct when addressing a group to return to their seats upon finishing their conversations.
When you've finished your conversation, please return to your seats
After you are finished chatting, please make your way back to your seats
Both 'after' and 'when' can be used to form this instruction. However, with a group of people - especially a large group, if you ask them to 'finish your conversation,' obviously some may finish quickly, while others may take a very long time and keep the rest of the group waiting. What often happens is that the person conducting the activity simply issues an imperative instruction such as:
"Please return to your seats now." Possibly, notice of this impending command may come a little earlier: "I'll be asking you all to return to your seats in about one minute."
'after（～の後）'と 'when（～したとき）'のどちらも指示を出すときに使うことが出来ます。
しかしながら、特に人数の多いグループの場合 'finish your conversation'（会話が終わったら）を使うことが出来ます。
間違いなく、早く終わる人もいれば、時間がかかりグループの他の人を待たせる人もいます。
グループ活動などを指導している人たちによく起こる簡単な問題は命令的な指示です。
例えば、
"Please return to your seats now."
（今すぐ席に戻ってください）など
もしかすると、差し迫った指示などは、少し早めに出した方がいいのかもしれませんね。
【例】
"I'll be asking you all to return to your seats in about one minute."
（大体1分以内に自分の席へ戻ってください）
Please return to your seats once you have completed the activity.
Once you have finished speaking to your activity partner please can you return to your seat.
"Please return to your seats once you have completed the activity." This politely tells them that when they have finished talking you would like them to return to their seat.
"Once you have finished speaking to your activity partner please can you return to your seat." This is also a polite way of telling them when they have finished the activity they must go back to their seat and sit down.
"Please return to your seats once you have completed the activity."
（この活動が終わったら席に戻ってください）
これは丁寧な表現で、話し終わったら自分の席に戻ってください、と伝える表現です。
"Once you have finished speaking to your activity partner please can you return to your seat."
（パートナーと話し終わた時点で、自分の席に戻ってください）
これもまた丁寧な表現で会話の活動が終わったっら席に戻って座ってください、と伝えるもう一つの表現です。
Return to your seats when you are done talking to the person you're talking to
Go back to your seats after you finish talking to the person you're talking to
To go back to something is also known as to 'return' so you can say 'return to your seats' or 'go back to your seats'
you can use both 'when' or 'after' they both suggest you are done or finished with what you are doing
どこかに戻ることは 'go back to' 以外に 'return' ともいいます。
ですから、「席に戻る」は 'return to your seats' あるいは 'go back to your seats' と言えます。
'when' も 'after' も使えます。どちらも「今していることが終わったら」のニュアンスです。
Once you have finished your conversation please can you return to your seat
Please go back to your seats after you've finished speaking with your partner
"Once you have finished your conversation please can you return to your seat" using the term 'once' in this context is referring to 'after'.
"Please go back to your seats after you've finished speaking with your partner" is a very casual and polite way of asking this particular question.
"Once you have finished your conversation please can you return to your seat"（会話が終わったら自分の席に戻ってもらえますか）
= 'once' はここでは 'after'（～の後）の意味です。
"Please go back to your seats after you've finished speaking with your partner"（パートナーとの話が終わったら自分の席に戻ってください）
= これは非常にカジュアルで丁寧な言い方です。
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