This sounds very much like Robert A. Heinlein's Tunnel in the Sky. (One of the "juvenile" SF novels that he wrote for Scribner's back in the 1950s, aimed at a target audience of teenage boys.)
As you recalled, the starting premise was that a final test of survival skills involved being dumped on the surface of an Earth-like (but wild and uncolonized) planet to see if you really had learned how to fend for yourself when you were literally light-years away from the benefits of modern civilization. One of the things that made it a stringent test was that you were given no idea of which planet had been selected, nor what sort of climate you would be placed in (tropical, or arctic, or somewhere in between?), nor of what the nastiest local lifeforms looked like and acted like. So you had to be ready for anything!
The vague warning that you mention was to look out for "stobor." At the end of the book, it was finally explained by one of the instructors that the word didn't mean anything in particular! It was deliberately cryptic, a term that had never been mentioned in any of the textbooks or classroom lectures. The point was to make students nervous enough that they would remember to stay alert for some mysterious organism, appearance unknown, which might want to eat you (or cause other serious problems).
As you say, something went badly wrong on Earth's end after the students had all been dumped in the same region on the surface of the planet. The characters came from several different classes at different schools -- as I recall, some were high school students (i.e. teenagers of various ages), and some were college students, who were mostly up in their twenties, I believe. As you say, after weeks had gone past without anyone opening up a space-warp and summoning them to gather around it and go home, it became painfully clear to the students (those who were still alive) that they were stranded, with no telling for how long, and they had better get together and organize a village, complete with some sort of government structure, for mutual protection.
One thing that your memory had blurred, though -- the main viewpoint character (Roderick "Rod" Walker) and all the others were not stranded on that strange new world for as long as fifty years. I believe it was closer to three.
Here's an excerpt from Amazon to demonstrate that we're almost certainly remembering the same book.
It was just a test . . . But something had gone wrong. Terribly wrong.
What was to have been a standard ten-day survival test had suddenly
become an indefinite life-or-death nightmare. Now they were stranded
somewhere in the universe, beyond contact with Earth . . . at the
other end of a tunnel in the sky. This small group of young men and
women, divested of all civilized luxuries and laws, were being forced
to forge a future of their own . . . a strange future in a strange
land where sometimes not even the fittest could survive!
