This actually ties back to the multiple time lines theory of Back to the Future (explained in the second movie), and in this case it gets a little more spaghetti'd.
In the original time line established at the beginning of the first Back to the Future movie, Doctor Brown never traveled back in time, Clara died, the ravine was named after her and all the kids want their teachers to fall into it.
Jump forward to the end of Back to the Future 2. Doctor Brown has gone back in time and created a new time line, which Marty is now in (counters Bttf2 a little). In this timeline Emmett is asked to pick up Clara from the train station (just like when Marty is there), he does and he and Clara fall in love shortly before Emmett dies. While we never see a map of the ravine in this new time line, I would venture to say that it was not named the Clayton Ravine.
Jump forward (backward) a little more to when Emmett and Marty are scouting the tracks. Clara is on the same path as the original, original time line. No one picks her up, she tries to drive herself home and is on course to die. However, this time Marty and Emmett save Clara at the brink, once again altering history and creating a new, new, new, new, new... time line.
Go to the end of the movie and Eastwood/Marty falls into the ravine and creates (you guessed it) another new time line. The ravine is now called Eastwood Ravine and Clara and Emmett live happily ever after.
When Marty comments that the ravine was called Clayton Ravine, named after a teacher who died in it, he is remembering history from the very first, original time line, from the start of Back to the Future 1. The moment Emmett went back to 1885 he started to change history but Marty wasn't schooled in that new history, so he never learned it.