Doesnt aluminum and "The works" liquid plumber do it? Or is that another gas used in the bottle bomb?
Do it in class, say, "For my experiment, I just put some aluminum in here, and some of this liquid. First I mix these two" Screw cork on, place on table, "than I just need to go and grab some alkaseltzer over here" Have the alkaselzter sitting about 10 feet away, than while you go and get it, your bottle will explode, and you turn around and say, "Holy Crap, what did you do to my experiment!?!?!" To the kid sitting closest.
lol, that was probably stupid, but w/e.
Or, you could go to walmart, go to the paintball department, Buy a 20oz CO2 tank, and bring it in to class.
"Here, Mr. whoever, here is CO2, contained in a small bottle at about 2,000 PSI, so dont drop it."