However, they are not necessary compulsory or to be graded by exam as long as students can have an opportunity to study through these art-based subjects.
I wish you could include your own opinion in the introduction of your essay.
It is generally thought that there might not be always...
This paragraph is quite off-topic to me. I think you should focus on whether arts-based subjects should be compulsory instead of how teachers evaluate their students' performances.
In my view, let students still have some challenges in art-based subjects, but not give mark on their work.
Again, this is not aligned with the prompt given to you. Whether teachers should grade the work of students or not is not what the topic asks you to give your opinion about.