Because of severe dysmenorrhea, I want to ask for leave from the old class. How should I write a leave note?

From your description, we can see that you are very sad now. If we continue to work, there will be no good efficiency. Therefore, when you ask your boss for leave, you can directly explain your situation. After all, this is human nature. When no one is sick, you don't have to be afraid. Your boss will not allow you to leave. After all, he must be a reasonable person if he can do this position. He won't see you insist on going on in such an uncomfortable situation.