Were You There?
Actually the teacher, if she or he actually knew it, is being thrown into the briar patch. This is an excellent opportunity to give the students some of the basic reasons why scientists started about 200 years ago to believe that geological evolution is basically true.
Ask the kids if they have seen the TV show CSI, which stands for Crime Scene Investigators. Most of them have; I understand that the show has made forensic science a popular career ambition. The investigators look at all sort of clues, then use their knowledge of science to try to figure out what might have happened to leave such clues. Often (or, in fiction, always) they can arrive at a scenario such that there is no other way that all these clues could be the way they are.
I don’t suppose many crimes are solved by geological principles, but a lot of historical puzzles can be solved. Consider a river flowing to the sea. The water will carry dust and sand, and may have enough energy to push gravel and stones along the bottom. As the water arrives at the ocean the path widens, so the water slows down. Now it no longer has enough energy to move the stones, so they collect near the shore. Eventually they harden into the sort of rock called conglomerate. Farther from shore the water is moving slower, so the sand falls out, eventually to harden into sandstone. Farther still, the dust settles toe the bottom to form shale. Far from the river mouth the water is quiet, and plankton grow in the upper few meters. As the plankton die, their calcium carbonate skeletons drift to the bottom to form a calcium muck that eventually becomes limestone. (This doesn’t take all that long, as geology goes. People have found limestone containing bottle caps.)
The interesting part starts when the land starts to sink, or sea level starts to rise. Now the river mouth is farther inland, so the near-shore conglomerate is also farther inland. When the sinking goes far enough, the sand will settle out over the conglomerate that was deposited earlier.
More sinking leads to shale over the sandstone, then to limestone over the shale. If the land now starts to rise, you see the reverse sequence: shale over the limestone, sandstone over the shale, and conglomerate over the sandstone. Now some of the layers will be out of the water, subject to erosion.
A lot of this happened in forming the Grand Canyon in the U.S. A detailed description of the major layers is here. Actually, each of the major layers has lots of sublayers, hundreds in all. Note the sequence from the Tapeats Sandstone to Mauv Limestone. This is classic. Other layers seem to be out of sequence. Geologists explain this as a result of the rock not being underwater at some time, maybe with upper layers eroded away. Creationists will cry “Fudging!”, but often the surface below the “missing” layers will show footprints, and maybe actual evidence of erosion. As almost always happens in places where there are many layers of rock, different layers have different fossils, and the sequence of fossils is much the same in the Grand Canyon as in other locations.
This page gives lots more history of the Canyon. What geologists mean by "history" is a story about what might have happened in the past in order to produce the sequence of rocks that we actually see. There is sometimes considerable debate in cases where there is more than one possible explanation; geologists then look for more details, maybe comparing these rocks to comparable rocks in other parts of the world, where maybe there are more layers that can give more detail.
Another source of controversy is an explanation that doesn't correlate with anything else. This is known as an "ad hoc" argument, and is widely regarded as feeble. Things should hang together. The earth has only one history, although conditions vary from one place to another.
Ken Ham relies on "someone who was there". But lawyers know that eye-witness testimony can be highly unreliable. Tests and actual cases have shown that witnesses get faces wrong, and sometimes get sequences of events wrong. Written accounts are particularly unreliable, if you can't cross-examine the witness. (Try casting doubt in a public school classroom on the credibility of Sacred Scripture. ) On the other hand, geology offers physical evidence, just the sort courts like best. Blood samples and ballistics are the hard-evidence stuff of court cases, just as rock layers and fossils are the evidentiary stuff of geology.Some creationists claim to have scientific explanations that lead to young-earth conclusions, but geologists aren't convinced.
So when a student asks "Were you there", you can reply "No, but the rocks were!"