Story ID: Young Adult science fiction book, early 80's

I am looking for the title of a book from the early 1980's, available through Scholastic Book Clubs.

It is a science fiction story about a human boy who goes to a training academy on a desert planet. They are taught to survive in the environment with minimal technology, and the heroes are often drinking from spongy water-filled tubers they find in the desert sand. The planet has very little in the way of visible life but lizards and specialised plants.

The academy's graduation test is to be dropped in the desert and find your way back to the academy. The boy is dropped in the desert with an alien humanoid classmate with silver skin. As this happens, they realise an enemy warship is scouting the planet and has possibly taken the academy over. They must detour across even harsher terrain than normal to reach a city to warn of the impending establishment of a forward base on the planet.

After much hardship, there's success and the usual happy ending with 'you graduate even though you didn't really finish the proper test' device.