After youは「あなたの後で」→つまり「お先にどうぞ。」
Go aheadは"You go ahead."で「あなたが先に行って。」→「お先にどうぞ。」
After you.のほうが丁寧な表現ですね。
ご参考までに…
オンラインのレッスンならばお互いの顔が見えると思いますので、その時は黙って「どうぞ」というジェスチャー(手のひらを上にして、相手に自分の手を差し出す)だけでも大丈夫だと思います。
If you were in the middle of an online English lesson and you and your teacher started to speak at the same time, and you would like the teacher to speak first you could say a variety of things. For example, you could say something like, "Go ahead", "After you" or "I am sorry, would you like to go first?". All three of these examples will allow the teacher to speak first.
オンライン英会話レッスンで、先生と同時に話してしまって、相手に先に話して欲しいなら、これはいろいろな言い方ができます。
例えば:
"Go ahead"(どうぞお先に)
"After you"(どうぞお先に)
"I am sorry, would you like to go first?"(ごめんなさい、先におっしゃいますか)
どれを使っても、先生に先に話してもらうことができます。
In any situation, when you wish someone to go before you, you may say, "After you!" or "Go ahead." This may be for example, in a physical sense when people are going through the swing doors of a railway station - or alternatively , in non- physical situations when people are speaking at the same time, or perhaps picking up items in a sale at a department store. When speaking, you could also ask the other person to 'please finish' what they started to say.
You go first. - this is one way of saying that your teacher can speak first.
Your turn. - people "take turns" speaking, so you can tell your teacher that it is her turn to speak by saying "Ok, your turn."
The most polite way to "take turns" is by saying "After you."
You go first. - お先にどうぞ。先生にこういうと先生が先に話してくれます。
Your turn. - あなたの番です。人はtake turns(順番を守って)話をしますので、先生に"Okay, your turn."と言うこともできます。
After you.-一番礼儀正しいのが、こちら。お先にどうぞという意味です。
If you speak at the same time as someone it is always good to apologise although it does happen sometimes, to let the person know you want them to speak first then you can add , 'go ahead', 'please continue' or 'after you'
In a situation where you and someone else starts speaking at the same time, you could use any of these phrases which asks them to speak first. In England, if this happens and someone wants you to speak first they may just say "no, please" and then not speak (this is often said with a nod), and that also means that you should speak first.
If you want to tell a teacher that they can speak first, if you start speaking at the same time, you can say:
"After you."
"Please continue."
"Go ahead."