Just say I'm a little overwhelmed. If I continue to be a class representative, it will be bad for the class and my grades will not be very good.
Be weak. If you feel embarrassed to say it in person, you can write an application.
Generally speaking, the teacher will consider your situation and promise you unless you do well. The teacher likes you very much and won't let you go.
Dear teacher:
I'm sorry to tell you that I have to give up this job. I didn't finish the work well. I'm very sorry about that.
Teacher, please forgive me, I have no choice. I feel tired. I think someone will do better than me.
I like this job, but I don't think I am suitable for it. Work is a good job, but I am not a responsible representative.
Although I don't want to continue this job, I won't give up English. I will try my best to learn English. If one day, the teacher thinks that I still have the ability to be your representative, I will be very happy and grateful. If then, I will be happy to be your representative again.
Thank you for your understanding.
Your students: ~ ~ ~ ~ ~ ~
Time: