Our theatre group has used fog machines multiple times. I can't claim to be an expert on fog, but our fog mix has always been basically water vapor. We always announce we are going to use a fog machine before the show begins and place a note in our program just to air on the side of caution.
I don't believe there are any laws governing the use of fog/haze. Just be ready to be able to clear the fog for after it is not needed. When we did the Wizard of Oz we used fog for when the wicked witch entered and exited and at one time we had so much fog out on stage that it lingered a little too long. Fortunately, intermission wasn't too far off.
There no "Laws" in re fog/haze. There ARE codes and regulations. Check with your local AHJ which is probably the fire marshall. There are also contract clauses and stipulations for various unions and and individual performers. Fog/haze with today's machines and fluids is a relatively benign effect. Usually the most serious issue is how the venue's fire alarm system reacts to the effect and what steps are in place to handle the effect. Some older methods of producing haze/fog, such as Sal Ammoniac are now recognized as being hazardous to the health, and are no longer used but, sadly, actually not specifically "illegal".
There are basically 3 types of atmospheric effects:
1. Heavier than air groundfog, A low, dense ground hugging fog that flows down hill, usually produced by chilled vapor. Dry Ice, LN2 and chilled fog are the most common.
2. General area fog. Fog that tends to hover from the ground to 10' or so high. Density and height are determined by the machine operator and the director's wishes.
3. Atmospheric haze. A relatively light particulate "mist" often not visible unless a beam of light is shining through. Usually used to enhance lighting effects.
School Theatres have and will continue to use fog/haze effects. Just check with your local authorities for any restrictions with the people who manage/maintain the HVACsystem to make sure there are no problems/conflicts. HTH.
No laws exist against using haze and fog in a school performance space. You'll want to be certain that there aren't particle detectors in your HVACsystem or infraredsmoke detection sensors attached to your fire protection systems. If someone on your maintenance staff doesn't know whether or not those components exist in your facility, arrange a test.
While the particular logistics may differ from system to system, what I did two years ago with one of our spaces was had someone on our Building & Grounds staff notify our alarm company that we were performing a system test and that they should not forward any alarms on our system to the local fire department. Then we turned on the four or five foggers/hazers we had until we smoked the place up far more than we ever actually would for an event until we were sufficiently certain there would be no false alarms as a result of our use of fog.
I can't recall whether or not we had the fire marshal on site for that or not. They aren't necessary for the test, but we may have invited them there anyway. Should they be there for the test, also be sufficiently certain that there wouldn't be a false alarm, and then a week later there is a false alarm, they'll have a lot more sympathy.
Also make certain that in the curtain speech or playbills include a note about the haze effects. Most people who complain about atmospheric effects have psychosomatic reactions -- I think this is smoke, therefore I will cough, wheeze, and complain of my allergies and blame it on the smoke, but some people with chronic respiratory illnesses may have legitimate reactions; those people know who they are, and most can prepare themselves appropriately by using an inhaler, but those who cannot will know when to exit the theatre.