how can praise not be used in class

Answer 1

? Do you know what the question is trying to ask?

Answer 2

Are you asking when praise would not be used in a classroom?

You wouldn't praise a child if he/she got an incorrect answer to a math problem, for example.

Can you think of other examples?

Answer 3

You can't use it when it affects others negatively.
You can't praise a white girl for having such light skin.
You can't praise someone for a high grade when they cheated.
You can't praise a kid for having a mom and dad who helps him.