This is Space Winners (1965) by Gordon R. Dickson.
They were humanity's hope - the first young people selected to leave Earth for study in the Galactic Federation.
But something went wrong in deep space. Terribly wrong. Suddenly Jim Rawlins, Ellen Bouvier, Curt Harrington, and the squirrel-like Alien philosopher Peep were castaways, stranded on the Quarantined World of Quebahr - with no training, and little hope of rescue. Between them and the Federation's emergency beacon were primitive Mauregs, aggressive Walats, lizard-like Noifs - plus danger, conspiracy, and the mystery of an impending high-tech war on a backward, peaceful planet.
Without warning, the future of two worlds - Earth and Quebahr - depended on three young humans' ability to adapt and survive.
Here's a brief excerpt from the book where Peep introduces himself to the teenagers:
"Honored! Honored to make the acquaintance of you all!" cried the Alien, beaming. "Let me introduce myself, an erring and faulty student of nonviolence. I am an Atakit, from Juseleminopratipup. My name is Panjarmeeklotutmrp!"
Jim, Ellen, and Curt stared at each other and back at the Alien.
"Pan ..." began Jim, hesitantly, after a second.
"Oh, call me Peep. Call me Peep!" boomed the Atakit, exuberantly. "All humans do. My unfortunate name is impossible for them to pronounce."