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Dear teacher:

Because I can't fully absorb what I said in math class, I find it difficult to learn. Although it took a lot of effort, it still didn't work. In the long run, I gradually lost my enthusiasm for going to school and my motivation for learning. I hope I can concentrate my limited energy on the subjects I am good at, so I can't apply for math class and let me study by myself.

Applicant: XXX

Time x year x month x day

If you just don't want to take a class or two, ask for leave.