The key answer of exercise 23

This week we're having a no school rucksack week. We do this once a year. We use a small bag for our snack. But we leave the big one at home. Our text books and notebooks stay at school in the classroom. We haven't got lockers. They sit on the teachers desk. We also leave our pencil case with all our pens and pencils and highlighters at school, under our desk. The question is: Why don't we do it every week?