1. Sincere apology: apologize to the teacher and admit the mistake. For example, you can say to the teacher, "I'm very sorry for the inconvenience." I know I should ask for leave as soon as possible, but I really didn't realize that my illness would be so serious. "
2. Explain the reason: explain to the teacher why you don't take time off. For example, you can tell the teacher: "At first, I didn't think my illness was serious, but then my child's illness suddenly deteriorated, and I realized that I needed to call in sick as soon as possible."