I'm a college prof, not secondary (where I think this is aimed)--but I "let" my students take mental health days. All in all, my policy is this: You're an adult, allocate your time as you need to; know that generally the less time you allocate to this course, the worse your grade will be." I don't ask for a doctors note or any documentation generally because that's just fucking stupid. The only time I might intervene is if the student has extended absences due to illness, and then I'll approach them about getting the university involved so they have a bit of protection at that level.

And on that note, I've also taken mental health days and openly called them that. It's important that if we want to change the way we think about illness and work in this country that we lead by example.