Depends on if you're actually discharging extinguishers or not... here's the basic rundown of what to highlight:
Most civilians will have just Dry Chemical, while most fire departments carry at least Dry Chem, Water Can, and CO2. If you go to a government/military/etc. installation you might encounter Halon.
If you demonstrate a CO2, make sure EVERYONE has protective gloves on, and the people in the immediate area have protective gear on. You can get some nasty injuries if you're not careful around those things.
This should be obvious, but make sure you're outside and the class stands upwind. While inhaling a few breaths of dry chem won't kill you, it should obviously be avoided.
Make sure to touch base on the 3 most common types of fires (A, B, and C) and to show the identifying symbols. Fairly self explanatory. One quick tip that usually is not mentioned: if you have a Class C fire, simply shutting off the power usually turns it into a Class A.
Always make sure the students lay the extinguishers down, and don't leave them upright. If someone forgets to leave a pin in, you can have an accidental discharge if it falls over. Additionally, it's not so much a threat these days, but if one falls over just right and breaks the handle off or another part of the discharge mechanism things could get messy at the least, dangerous at the worst.
Stress this one rule: Never turn your back on the fire (or the smoldering remains of one). Always back away from a fire unless it's a SHTF moment and you're running for your life (if you hear the roof starting to collapse, for instance). Also, if your back is turned, it can rekindle, spread, and sneak up on you.
If the fire extinguisher is out of date, replace it. No exceptions.
If your car is on fire and it has a magnesium block engine (I find Volvos tend to have these more than other brands), do NOT put water on the engine. Get a dry chem extinguisher. Put water on a magnesium fire and you're going to have the fire intensify, and potentially flash in your face.
Remind them that having a fire extinguisher does NOT make them a firefighter. While it's good to know how to use a fire extinguisher, their job is not to put out the fire. Thats what we're here for. If you have a wastebasket on fire cause someone dropped a cigarette butt in it, go ahead and put it out before the whole building catches on fire. If you have anything bigger though (an entire wall, a room, etc), don't even bother. Sound the alarm and get out. We'll take care of the rest. We don't put on fifty pounds of gear and SCBA to look good, and we don't carry a 100+ gallon/minute fire hose for fun. A fire extinguisher won't put out a house fire, and trying to do so with one will just get you seriously injured or killed.
Finally, make it fun and keep it hands on. They're more likely to retain info that way.
I could go on some more, but I figure that's enough for starters. PM me if you need any more info, or have a specific question.