French students K-12 were back to school recently for two weeks, and now are out on break until September, as planned. We might see what becomes of their experience, but of course they have managed to "mitigate" the virus to this point effectively. It's still around though.
Staying "closed" longer does mitigate better, no doubt, but will the area under the curve be different? Sweden, as often noted, has not been a massive disaster, worse than the neighbors, but still not a disaster.
Area under the curve, it's important. Not overwhelming the health care system is critically important, and we're nearing that point in some states.