"Speciation" in the Darwinian sense of one species gradually changing by selection into another has not been observed and no examples are known.
Of course, it's wrong, but even better than reading about examples in scientific literature would be to demonstrate speciation in a high school classroom petri dish.
So here's my idea - take a bacteria species that cultures easily and normally lives in pH-neutral environment medium, not very acidic and not very alkaline. A teacher could demonstrate this by transferring bacteria cultured on a neutral petri dish to higly acid and highly alkaline dishes, where the bacteria quickly dies.
Now the invention that someone needs to make - a petri dish shaped like a long rectangle that has an acidity gradient, so that it's very acidic at one end, gradually changes to a neutral pH in the center and highly alkaline at the far end. Students transfer bacteria to the neutral pH location, and then wait. Within a time period of days/weeks, the bacteria should, through natural selection, gradually evolve and grow into areas that are increasingly more acidic or alkaline than the parent population could survive in. At the two ends of the petri dish, the students will have created two different bacteria species that are reproductively isolated from each other because they live in incompatible environments. The students can prove that by transferring bacteria from the alkaline end to the acidic end and vice-versa, and they'll observe that the transferred bacteria soon dies. Speciation as described by biologists, for all the students to see right in front of them.
Several additional notes:
Biological education isn't my field, so there may be difficulties with this in practice, but it seems doable.
The idea of a species* is a little squishy when it comes to bacteria, but even if one doesn't accept the species concept for bacteria, the experiment does simulate what happens in nature and lets students see natural selection in front of them.
If some bacteria survive as one long colony of bacteria from the acidic end to the alkaline end, the students will have created a ring species, which is itself a fascinating example of speciation.
Some creationists now accept speciation, and say all the species biodiversity on earth could have developed out of the tiny number of species that could fit into Noah's Ark. They really believe in Noah's Ark. These people will not be affected by this bacteria experiment, but when creationists are forced to start saying the polar opposite of what they had previously been saying, they're going to lose some supporters. The Noah's Ark Darwinists also have all kinds of additional problems, from speciation in slow-maturing vertebrates to zero support in the fossil record.
*UPDATE: changed language from "biological species concept" to "species". See comments for details.
UPDATE 2: I'm about to lose Haloscan comments into the void, and since there a just a few here, I'm going to append them:
key: science, politics, evolution