Q&A for How to Tell Your Teachers You Won't Be Here over Email

I would suggest something like "Expected Absence" or "Anticipated Absence" because it will allow them to gauge from the subject of the e-mail that it is important and you are attempting to be respectful by alerting them in advance.

Hello, class. I will not be at school today because of a family emergency. Work on the assignment titled ("name of assignment"). The substitute will help you work through it. Best Regards, (name of teacher).

Dear (name of teacher), I didn't quite understand the lesson you taught on (day of the week) regarding (topic of presentation). Could you please explain it to us again? Thank you, (your name)

Hello Mr. Robertson, I am sick today and can't attend school. Could you please forward to me today's reading and homework assignments? Thank you for understanding. Sincerely, Jessica.