I just learned a new meaning of 'take it up' today. When a teacher gives answers to all the questions on a test, we can use 'take it up.' I made up a sentence myself as follows:
"The teacher used almost an hour to take up the exercise."

Does this sentence sound natural?

Thank you and good evening. :)

Question

I just learned a new meaning of 'take it up' today. When a teacher gives answers to all the questions on a test, we can use 'take it up.' I made up a sentence myself as follows:
"The teacher used almost an hour to take up the exercise."

Does this sentence sound natural?

Thank you and good evening. :)

Answer

Hi prudent260, according to m-w.com, there are at least nine ways to use the phrasal verb "take up." (https://www.merriam-webster.com/dictionary/take%20up) The example you wrote would be incorrect, but alternatively you could use "take up" to describe the time that she occupied to do the exercise - "The teacher took up almost an hour of the class with the exercise."

Answer

No. And it’s not clear what ‘When a teacher … use ‘take it up’’ is supposed to mean 
either.

‘Take up’ usually means ‘use/fill’ or ‘start’

It takes up 2 hours/10 meters.
That’ll take up my entire weight allowance. 
Being a parent of triplets takes up all of her attention.

I took up guitar. 
I’ll take it up with the administration. (Bring it to the attention of)

Answer

Robert gave an excellent answer.

Answer

I've never hear 'take it up' used that way.   Check robert's link for the various uses.