I'm not sure what kind of guppies we are talking about here, but male fancy gups tend to run about 3.50 each around here with the females not too far behind. Platies would probably be cheapest because you would be able to get a variety of colors ("breeds") for anywhere from .99 to a few dollars depending on what you want to spend.
I guess the real question is do you want a pair of cool fish (i.e. the convicts) or a swarm of little guys (i.e. some type of liverbearer)? You could make an argument either way about what students would find more interesting to look at. Conicts would be more illustrative of breeding since you'd see the eggs while the livebearers just get fat and then one day there are little guys swimming around.
The comment about kids being stupid was over the line. I'm not in high school anymore, but when I was I certainly would have taken offense to a comment like that. What do you think they're going to do, stick their hands in the tank and yank something out? Give me a break.