old (possibly 50's?) science fiction book about a group of kids stranded on a planet when their teacher becomes incapacitated

[atomsplitter5]

i seem to remember that the book cover was blue and had a planet featured on it. the word "hidden" or "mystery" may have been in the title. the plot is about a group of kids of various alien races that are taken to a planet for a test and their teacher becomes incapacitated (it's later revealed that this was part of the test). if i remember correctly it had something to do with a hidden ocean or river on the planet. i do remember pretty clearly that they had to solve some mystery about the planet and that it was time critical that they do so.
i know it's not a lot to go on but i'm hoping somebody here can help me

 

[Vince W]

This sounds somewhat like Tunnel in the Sky by Robert A. Heinlein but there aliens involved in the test and clearly it doesn't contain your title words.

 

[hitmouse]

This sounds somewhat like Tunnel in the Sky by Robert A. Heinlein but there aliens involved in the test and clearly it doesn't contain your title words.

My first thoughts too, but it doesnt quite fit.