Is it ok to write
I am writing to inform you that my son will not be attending school for three weeks starting from 21.04.2008, due to a fracture which he suffered from a twist [omit] of his left ankle.
Our aim is that this absence from class not affect my son's academic performance.We intend that during this period he will not be affected by his absence. In order to ensure this, we will make every effort to ensure that he completes his schoolwork while he’s away.
Thank you in anticipation of your help.