A lot of girls get their first period during school hours. It could be during a boring lecture, recess or worse, the exercise period. The first time can make a girl feel super awkward but having an adult to make the situation seem comfortable helps a lot.
Kristin Heavner, a teacher, gauged this exact problem and came up with a solution. The simple solution presented as a Facebook post got over 10K shares and the comments section has women of all age groups appreciating Kristin's initiative.
Upper elementary and higher teachers (if you are a person uncomfortable with the idea of folks menstruating, you should prob not read ahead, but then really ask yourself why you are uncomfortable with it because seriously, about half the population of the world menstruates...)