Well, aside from setting smoke alarms off and telling a bunch of Cub Scouts to light up, I don’t see anything wrong with that…
Back in high school one of my classmates did a presentation on lasers and he used a simple spray bottle with water (you can probably find a small one at your local dollar store) so we could see where the beam was.
You could even give spray bottles to the scouts, since it’s just water.
He tried that, see post #6.
You may be able to get permission to turn the fire alarm off. One of the live music venues I work in had the “wrong” kind of fire alarm installed and we could not use fog or haze. After many complaints from touring acts it was discovered the the Fire Marshall will let the alarm system be off for the evening. It requires a (cheap) permit and there are some rules but they let us fill the venue to capacity(1750 ppl)and run shows with no noticeable change.
I would ask the Fire Marshall first and then the school, couldn’t hurt.
Good luck
Capt
A school cafeteria should be large and high enough not to concentrate fog from a machine. A small fan set nearby can create a breeze that will push the fog through the laser beams.
Dry ice in a bowl on a table will spill over the bowl and table, creating a curtain of “smoke” to play with.
If you can use a hotplate, boil water in a pot; the resulting steam is teeny tiny droplets, and if you wave the laser’s beam back and forth - in the dark - you’ll see them lit individually, and you can see a “slice” of the steam.
Dry ice in a milk jug or similar gallon-sized container can create a smoke jet. Punch a hole near the top about 1/8" to 1/4" in diameter. Add crushed/broken dry ice, fill with hot water, and screw on the cap. You will have a smoke jet that goes out 3-4 feet.
Do this in the dark about 30 feet from the kids. Give one or all a laser, and have them find the fog by sweeping the beam around.
If you really want fog that won’t rise, look up fog cooling projects on the interweb.
The laser from my pistol is visible at the indoor range on an intermittent basis. So…you could fire few hundred rounds of ammo in a confined environment first?
Make sure it’s lead-free ammo. Or at least Total Metal Jacket. 
Found a reasonable compromise. We’ve got an ultrasonic mister which’ll do, plus, in a clear vessel you can show the laser curve in water. two for the price of one!