Hi Visen, all of these are correct, but I've added some comments to help you inthe future.

Tenses

Hi, are these sentences correct?

I've taught a lesson today. (Grammatically yes, but we rarely say this)
I taught a lesson yesterday. (This is more commonly said)
What did you do yesterday? - I taught a lesson.
Look what you've done! - Sorry. Hold on, I'll get you a rag.
I've been teaching maths since 1996.
When he came (We usually prefer "arrived") I had already left the classroom.
She had been teaching history in school until she got married.

tenses

Hi. Are the sentences correct?
I've taught a lesson today.   incorrect
I taught a lesson yesterday. correct
What did you do yesterday? - I taught a lesson. correct
Look what you've done! - Sorry. Hold on, I'll get you a rag. correct
I've been teaching maths since 1996. correct
When he came I had already left the classroom. correct
She had been teaching history in school until she got married.correct