I can’t believe that it’s already January 20th.

It’s been 20 days since the New Year started.

Anyways, I have something funny to share with you.

Today, I asked my students if they have ever watched any cartoons in English.

(They are 10 years -old)

All of them said “Nno.”

Then one of them said that he has watched “Demon Slayer.”

But I know what that meant, e. Even though he said it in English, it’s a Japanese anime.

All of the students gotwere surprised, and they started asking me how much I knew about it.

They asked me if I knew Akaza (one of the characters),.¶
¶
I said no.

別の選択肢：I can’t believe that it’s already January 20th.

It’s been 20 days別の選択肢：20 days have passed since the New Year started.

(TAll of theym are 10 years old.)

(He said the title in English.)

PUNCTUATION: A period is needed.

They asked me if I knew Akaza (one of the characters), and I said no.

(They are 10 years old.)

It's a complete sentence.

All of them said, “No.”

comma before dialog quotations

Then, one of them said he hasd watched “Demon Slayer.”

The story is in the past tense.

But I knoew what thait meant, even though he said it in English,; it’s a Japanese anime.

The story is in the past tense. ("It's a Japanese anime" is a fact, so it stays in the present tense)
"that" may be correct grammatically, but it sounds odd. (I would use it to emphasize a meaning. For example, "Your mother called and hung up as soon as I said you weren't home. You know what that means.")

I said I knew that it was Kimetsuno Yaiba, but I’ve never watched it before.

You don't need "before" because "never" is enough.
You could add "before" if you were about to watch it with the class.

All of the students gotwere surprised, and they started asking me how much I knew about it.

If you wanted to use "got," you could say, "got excited."

They asked me if I knew Akaza (one of the characters),. I said no.

You could also use a semicolon, or add a conjunction after the comma.

I laughed so hard, and I was like, “No way, are you serious?!”

January 20th

I can’t believe that it’s already January 20th.

(They are 10 years old.)

Then one of them said he has watched “Demon Slayer.”.

The period isn't part of the anime title, so I think it makes more sense to leave it outside the quotation marks.

(He said the title in English.)

But I know what that meant, even though he said it in English,; it’s a Japanese anime.

I said I knew that it was Kimetsu no Yaiba, but I’ve never watched it before.

All of the students gotwere surprised, and they started asking me how much I knew about it.

"Got surprised" is an awkward expression.

I laughed so hard and I(I) was like “No way, are you serious?!”

You can omit the second "I". It may help the sentence flow better.

But the other students agreed with him.