You are living through a special time. It is so special that it is going to go down in history!
Never before in this country have schools been closed down completely for a long time. Even during the second world war, when children were evacuated from London to protect them from bombing, they still went to school in the small towns and villages which became their temporary homes.
Your diary might be read by future historians or school children so that they can learn about this time.
You could write about what is happening at home, what you do, how you feel, how other members of your family are reacting.
You could write about what aspects of school you miss if you are at home.
If you are still at school you could write about how it is different from 'normal' school.
You could write about any problems your family has with getting food and other essentials.
You could write about what you and your family are doing for exercise.
HOW ABOUT SENDING A PAGE OF YOUR DIARY TO CHALKHILL WRITERS?