Space Cadet — Robert A. Heinlein

Only the best and brightest -- the strongest and the most courageous -- ever managed to become Space Cadets. They were the elite guard of the solar system, accepting missions others feared, taking risks no others dared, and upholding the peace of the solar system for the benefit of all.
But before Matt could earn his rightful place in the ranks, his mettle would be tested in the most severe and extraordinary ways -- ways that would change him forever but would still not prepare him for the alien treacheries that awaited him on strange worlds far beyond his own.