This isn’t a binary thing: spread or no spread. You will have spread no matter what you do. The question is how much. If prevalence of the virus is high, then

This isn’t a binary thing: spread or no spread. You will have spread no matter what you do. The question is how much. If prevalence of the virus is high, then the risk of spread is higher. At a high level of prevalence any systematically repeating and mixing groupings of people (such as school) is pretty much guaranteed to lead to an outbreak, which will stop or at least seriously disrupt and hinder that system (not to mention the sick and dead people from the outbreak). At a low level or prevalence outbreaks will still occur, but only here and there. Some schools will have outbreaks and have to stop, but most will be able to keep going. Widely available tests with fast turnaround is also part of this because it allows outbreaks to be detected earlier, when they are much smaller, which limits the disruption each outbreak causes (not to mention the deaths and sickness). The point of all this is to have most of our normal activities and economy until a safe and effective vaccine is widely available.