Answer:
Dear Headmaster: I must explain to you the causes of my prolonged absence from the institute, since as of today I have not attended it for three weeks. The truth is that, as a result of the current coronavirus pandemic, 21 days ago my father began to manifest symptoms such as cough, fever and body pain, after which he decided to carry out a test to check the possibility of being infected with said disease. Three days later, the test gave a positive result, with which the whole family went into quarantine, including me.
Then, gradually we all started to feel the same symptoms, since as a closed family group we live in the same home and, therefore, we infect each other. So, 2 weeks ago it was I who began to experience symptoms, so I could not return to the institute.
Therefore, it explains my temporary absence during these weeks. As soon as I have the test to guarantee that I am no longer infected, I will gladly return to class.