It's for stool testing, used to detect digestive parasites in children (worms, etc).
Children are given stool sample envelopes to bring home so they can collect samples and are returned to the school for lab testing. If parasites are detected in the stool, the child is given deworming medicine.
Apparently, the girl forgot about it and asked MC for a bit of his poop so she could pass it of as her own. MC has never been called out for parasites over the years so she's confident his gut is healthy.
MC coincidentally found someone in the bathroom already pooping and asked for that person's poop instead (probably because he doesn't want to, and there's a quick source already available).
Coincidentally, it's the guy with parasites in their stool. (●'◡'●)ﾉ
Thanks ヾ(☆▽☆)
I don't understand chapter 16. Poop?