Gravel
This is how it looks on the floor in one of the classrooms I teach in today. Sure the kids were out in the woods, sure they dragged the dirt and gravel in because it was wet...but my point is,,,that was on Friday. This picture is taken on Monday on my third lesson.
Would YOU like to work on dirty floors like that? Well, neither do I.
It is not good for the kids (or teachers) to work in environments which are not cleaned from any aspect. First it is the normal health issue, allergies, asthma, etc, then it is the mental issue that you should have a pleasant place to work at. A dirty environment cause stress, not a sense of well-being.

On a happier note: Last week I got a cortisone shot in my left knee. That really helped. Normally my knees feel like they have some gravel in the joints as well and give me a lot of pain but now, lo and behold, the knees give me almost no pains now and I can actually walk (and look human doing it) the stairs in a normal speed.