There are very few documented cases of transmission in the school setting. Can't really think of one in fact. All the reports I've seen it's been pretty clear that the infection happened outside of school. True for employees, teachers, and students.
Have heard of parents out west sending their kids to school WHILE KNOWING that there had been exposure (without masks) to people who had tested positive. If kids and employees only wear face masks at school, that obviously increases the risk of exposure that results in COVID-19 outside of school quite a bit. Could well be other family members who are the source for the kids.
For other countries, the approach can be to keep schools open while closing or severely limiting the hours of restaurants, bars, or other businesses where adults gather to relax while drinking, talking, or singing. Of course, if those adults just move their gatherings into someone's home, then there is still plenty of opportunity for community spread.