That's not the worst case scenario, since the age of the Earth is geology which is not taught in High School
The worst case scenario is they teach that evolution is not the underpinning of biology, so we never get good biologists anymore and we end up with incompetent doctors, no medical research that means anything and a bunch of dead people.
Pasteur and Mendel didn't believe in evolution and they were contemporaries of Darwin.
Or even that more people do as many of the scientists that I now have done: decide that the Republicans are a bunch of stupid religious nuts and vote for anyone but the GOP. It's getting more common, unfortunately.
They tell me, if the GOPers are so stupid as to get simple science wrong, how can they be trusted on things like Social Security or the War on Terror; they'll bring their same lack of reasoning to these issues too. (To be fair, these guys didn't vote for Clinton either, for the same reason: they couldn't trust his judgement or statements.)
For example, there have been many cases, even on FR threads, of Creationists altering someone's writing. Why would I trust anyone who will not even quote published items correctly to act correctly in other matters? (Same argument against Clinton mutatis mutandis.)