Cases of COVID in school children have spiked in an extreme way since the holiday school closure (see chart "Positive results by age group: All Ontario" @ website below).

Schools being open is a public health measure that likely slows the spread of COVID. https://covid-19.ontario.ca/data 

This data matches what @MLHealthUnit and others had found indicating that over 80% of student cases (likely more) were linked to a previous COVID case at home, not at school.

It may seem counter-intuitive, but the data shows that students catch more COVID at home than at school.

One thing that we know about the current school shutdown is that many families don't have access to affordable child-care options, so are likely getting together with many students from other households, which is the pattern over the holidays that caused this spike in cases.