Tunnel in the Sky (1955) - one of Heinlein's "juveniles" from Scribners (that didn't see paperback form until 15 years after it came out.) I've read this and like it, but for a decent plot set-up, I rely on an excerpt from a review of that first paperback - by Lester Del Rey from IF January 1971:

"…It's the story of a bunch of young men and women (old enough to have children, so I don't consider them "kids") who are sent through a tube between or through or over the dimensions to another world as a test of their fitness to graduate from Survival Training. They can pick their own weapons and equipment but they're to know nothing about their destination in advance. They must survive there for two weeks before they can be returned. Those who live graduate; the others obviously have failed the course.

The going for the principals is tough enough for two weeks. But at the end of that time the pickup fails to appear. And they gradually realize that they may be stranded for years - or forever. They still know almost nothing about the dangers of the world on which they find themselves. But as best they can they have to find a way to pass the ultimate Survival Test - the survival of self and kind. Nothing is obvious from there on. But it's beautifully worked out. If you haven't read it, do so at once." [-Lester Del Rey]