Ok, that was exactly my point earlier. I thought that since some countries experience annual cicadas, the U.S. experiencing both periodical cycles simultaneously would be similar to what other countries experience every year. However, I was wrong. It turns out that the numbers for periodical cicadas are much, much greater than those for the annual types. So, indeed, just their sheer numbers will make them a spectacular event. Sorry for the misunderstanding. TIL
In my experience, this is because they conduct yearly x-ray examinations to detect tuberculosis and other lung diseases. Shirts and bras interfere with the image, so they ask students to bring a plain shirt to wear during the examination. However, if there's no other option, they may ask them to do the examination bare-chested. In my case, there was gender separation (not only in location but also in the time frame), but I can't say the same for all schools or age groups. Japanese schools are known for having students change into sports clothing in the classroom without gender separation up to 3rd or 4th grade, so I wouldn't be surprised if there are schools that don’t separate genders for medical examinations.