Different rules for indoors and outdoors. Indoors is trickier. The amount of time spent, the amount of people and the surfaces touched are all factors. I linked an article that I found useful for risk determination for simple day to day use.

Masks make a difference. Combined with social distancing (10 ft preferred) they can be very effective. Mandatory masks in public, along with social distancing really curtailed the asymptomatic spreading and was the turning point in Michigan. More testing and quicker processing of tests was the other major component.

Case in point: 2 hairdressers in Texas tested positive for COVID-19. They saw 150 clients. Both workers and clients were masked. No clients tested positive.

In schools, most of the discussion I've heard is for in the classroom - smaller class sizes, masks and ventilation. It will be difficult to control behaviors in hallways or on the playground.