The New York Times surveyed a group of pediatric infectious disease experts and their message was clear: It's OK to send kids back to school. But only if common-sense precautions are taken:
The 175 experts — mostly pediatricians focused on public health — largely agreed that it was safe enough for schools to be open to elementary students for full-time and in-person instruction now. Some said that was true even in communities where Covid-19 infections were widespread, as long as basic safety measures were taken. Most important, they said, were universal masking, physical distancing, adequate ventilation and avoidance of large group activities.
This is the part that bugs me: "adequate ventilation." What does that mean? How is it measured? Who decides if a classroom's ventilation is adequate? If a building is serviced by a commercial HVAC, that's fine. With little or no change, air conditioning systems will guarantee adequate ventilation. Unfortunately, a recent GAO study suggests that nearly half of all school districts have lots of classrooms with inadequate air conditioning systems:

And if a classroom doesn't have air conditioning at all? The EPA basically says you should open some windows and maybe use a few fans. There's not a lot more you can do.
But is that enough? It sure seems like we could use some quantitative guidance here.
POSTSCRIPT: It's also worth noting that although the pediatric experts believe COVID-19 transmission is minimal in classroom settings as long as precautions are taken, they also put a lot of weight on the negative effects of kids being out of school. To some extent, they're willing to accept a small amount of COVID-19 transmission as a price worth paying to get children back in their classes.