There were zero cases in my daughter's school too... until this week. Now two kids have had confirmed positive tests which has meant two separate year groups worth of children (including my daughter) are having to self-isolate until Boxing Day. She can't leave the house for 10 days because she potentially came into contact with the other child.
It's going to be f**king nightmare not even being able to take her outside for some fresh air and exercise for 10 whole days.
In terms of your child's school being closed... it's better to be preventative than to act after the fact, don't you think? All it takes is one positive test and it will effect you personally.