Fireballs are an excellent way to perk up a classroom. This video shows you how to create a fireball in a closed setting. The scientific reasoning may be low, but the "wow-factor" is phenomenally high.
The trick can be performed with a five-gallon water bottle, isopropyl alcohol and a match. Add a small amount of the alcohol into the bottle and swirl it around so it covers the interior. Make sure you pour out any extra liquid, we're not looking to blow it up, just burn the vapors. To energize the reaction you can blow into the bottle — the extra oxygen will help the fuel burn, though it's likely not necessary. Once you're ready to wow the room, drop a match (with tongs or a stick unless you like burnt fingers) into the bottle and watch the "foosh!"
ADDED SCIENCY EDUCATIONAL BONUS: Once the flame has disappeared, cover the top of the bottle to keep air from rushing back in. A hand works well, but make sure you've got skin protection. If vacuum left behind after the alcohol burns away the oxygen isn't filled with air again, the atmospheric pressure in your classroom (garage or backyard) will physically crush the now very empty bottle. Science is cool!