Sci Fi involving portals to distant worlds that leave people stranded far from earth

Question

Probably 1970-1990 timeframe. The book involves a group of youth being sent to a distant planet by a doorway/portal for a short survival course. The doorway fails to reopen and the youth are forced to survive on this world where there is no civilization. Eventually the survivors are rescued and told the doorway was unable to be reopened due to an explosion of a supernova between earth and the planet where the youth were stranded.

Answer 1

I believe you're thinking of Heinlein's Tunnel in the Sky. Quoting from the Wikipedia plot summary:

The final test of his Advanced Survival class involves staying 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 they are otherwise completely on their own. They are told only that the challenges are neither insurmountable nor unreasonable. (It seems that the mores prevalent at the time regard it as acceptable to expose high-school students to very concrete life danger as part of their studies; at least, Rod's parents make no objection at knowing they may never see their son again.) On test day, students walk through a Ramsbotham portal and find themselves alone on a strange planet, but reasonably close to the designated pickup point. Rod, acting on his older sister's advice, takes hunting knives and basic survival-gear, but no high-tech weaponry, on the grounds that the latter could make him overconfident. The last advice that the students receive is to "watch out for stobor".

On the second day a thief ambushes Rod and knocks him unconscious. He wakes up to find all that he has left is a spare knife hidden under a bandage. In his desperate concentration on survival, he loses track of time. Eventually, he teams up with Jacqueline "Jack" Daudet, a student from another class whom he initially mistakes for a male. When she tells him that more than ten days have elapsed without contact, he realizes that something has gone wrong with the portal that was supposed to recover them, leaving them stranded.

Quote from near the end of the book re the cause that matches the OP's recollection:

"How did it happen that they left us here? We were only supposed to be here ten days."

...

"It was the supernova, of course. Delta, uh -"

"Delta Gamma one thirteen," supplied Mac.

"That's it. Space-time distortion, but I'm no mathematician."