In Hungarian, there is a new-ish folk-song sung by school-children who dislike school whenever it snows to the tune of Twinkle, twinkle, little star:
Hull a pelyhes dinamit,
Robbantsuk fel a sulit!
Minden gyerek várva vár,
Hogy repüljön a tanár.
[irrelevant verses removed]
It would roughly translate to the following:
The fluffy dynamite is falling,
let us blow up the school.
Every child can hardly wait
for the teacher(s) to go flying.
Now this raises a few questions, like what is wrong with these children, but we aren't talking about that.
What explosive could potentially end up falling from the sky in a snow-like form? The less human intervention needed, the better (I mean, you could fly a plane over and dump it, but that's cheating).
It should be powerful enough to blow up a small school with less than a cubic meter of the stuff (a rough estimate of how much a schoolyard full of kids can collect and smuggle in without being apprehended).
Bonus points if you can make "snow"men or "snow"balls out of it. Extra bonus points if the snowballs explode. Even more bonus points if you can go skiing in it without blowing up.
This phenomenon is happening on a planet that looks like Earth, and has humans. So, blue sky, 1G gravity, and inhabitable by humans. If you want, you can put it on another planet, but the teachers need to be taken by surprise, so this is not a regular occurrence.