It is all random, all the time:
Education and health officials around the country have taken different approaches to testing students and staff members — and widely varying positions or whether to test them at all as more children give up virtual classrooms for in-person learning.
Source: Virus testing strategies, opinions vary widely in US schools
The big problem with mass screening of asymptomatic people is that when the prevalence of the disease in the sample group is less than the false positive rate of the test, then the majority of positive results will be false positives.
Rapid tests have a higher false positive rate. They are supposed to be followed by a confirming PCR test. But some places do not do that – even if you get yourself tested again and show its a false positive, their policies still mandate that you and your family quarantine for 14-days.
In the case of schools, as has occurred locally, when one student tested positive (even without symptoms), the entire class was sent home and told to quarantine for 14-days. This has ramifications for many people – and such testing is not a zero-risk activity because of this.
In some schools, the testing procedures are crazy:
Some districts, like Baltimore City Public Schools, use so-called pooled testing methods that combine multiple samples from students in kindergarten to eighth grade; a positive result leads to everyone in the pool being quarantined.
As Monica Gandhi, MD says, “At the same time, she said, false positives in school settings carry significant consequences when they cause a return to online learning.”
Ultimately, once again, the randomness in policies illustrates the randomness of epidemiology, and calls into question the foundation of the field itself.