This is Heinlein's [Tunnel in the Sky][1] unless he got seriously copied.  The incident with a kid who owns the dog and big gun and gets killed is right out of that novel.

From the [Wikipedia summary][2]

> Rod Walker is a high school student who dreams of becoming a
> professional colonist. The final test of his Advanced Survival class
> is to stay alive on an unfamiliar planet for between two and ten days.
> Students may team up and equip themselves with whatever gear they can
> carry, but are otherwise completely on their own. They are told only
> that the challenges are neither insurmountable nor unreasonable. On
> test day, each student walks through the Ramsbotham portal and finds
> him or herself alone on a strange planet, though reasonably close to
> the pickup point. Rod, acting on his older sister's advice, takes
> hunting knives and basic survival gear rather than high-tech weaponry,
> on the grounds that the latter could make him over-confident. The last
> advice the students receive is to "watch out for stobor."

Here's the part where Rod finds the dead guy:

> It looked mightily like a man on the ground and a child near him.  Rod
> reached, fumbled in his vest pack, got out a tiny 8-power monocular,
> took a better look.  The man was Johann Braun, the "child" was his
> boxer dog.  There was no doubt but that they were dead, for Braun was
> lying like a tossed rag doll, with his head twisted around and one leg
> bent under.  His throat and the side of his head were a dark red
> stain.

The only thing that doesn't match your recollection is the part about them not really being on another planet.  That doesn't happen in *Tunnel in the Sky*, but at one point Rod is convinced that this is the case.  Another kid points out the error in his theory.

  [1]: http://www.isfdb.org/cgi-bin/title.cgi?957
  [2]: http://en.wikipedia.org/wiki/Tunnel_in_the_Sky