Some people think that art is an essential subject for children at school while others think it is a waste of time.
It appears that you are missing a comma or two with the interrupter on the one hand. Consider adding the comma(s).
It seems that article use may be incorrect here.
It seems that preposition use may be incorrect here.
The noun phrase important art seems to be missing a determiner before it. Consider adding an article.
Don’t put your opinion unless you are asked to give it.
If the question asks what you think, you MUST give your opinion to get a good score.
Don’t leave your opinion until the conclusion.
Here are examples of instructions that require you to give your opinion:
...do you agree or disagree?...do you think...?...your opinion...?
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