Mankind is colonising the universe via instantaneous interstellar travel, accomplished through artificial wormhole- like "gates". Late teen hero Rod Walker is marooned with his classmates on an unknown planet when a "routine" ( but dangerous) high school survival training exercise goes dramatically wrong. The story follows their enforced transition to adulthood as they gradually band together to build a functioning society from the ground up, cut off from civilisation with no expectation of rescue.
I've read this book many times, as a child and adult, and have never failed to enjoy it for what it is: classic "juvenile" Heinlein. Whilst "Citizen of the Galaxy" it is not, it is on a par with "Time for the Stars","Farmer in The Sky", "Between Planets" et al in that it epitomises Heinlein's strengths and weaknesses.
On the plus side, originality of vision, a strong plotline, taut driving narrative, interesting minor characters realised with great economy and that overall indefinable feeling of authenticity that made Heinlein's future worlds uniquely real.
On the minus, a typical passive-reactive hero ( Heinlein's leading characters of this period all seem to have things happen to them, rather than initiating the action), the usual "aw shucks" (ie non existent) relationships with women, and the USA centred view of the world c 1959.
From the perspective of later years and his later work, you can see the stirrings of some familiar Heinlein themes, the relationship of the individual citizen to society ( "Starship Troopers" anyone? ), the superiority of self reliant individualism, the admirability of the military ethos. None of these however detract in any way from a cracking adventure story that should be an essential part of the reading list of any Heinlein fan.