ich würde es so schreiben:
Dear Mrs. XX,
I'm sorry that XX hasn't been to school the last two days.
My mum had a heart attack and we were all worried about her because she is 80 years old.
This is why we stayed at home in Carinthia these days.
I hope that this is not a problem for XX and that it's possible for her to write the test again.
Thanks a lot for your understanding.
With kind regards
lg Edel
