Ranger's apprentice. Book 1, The ruins of Gorlan

Will is small for his age, but fast and quick-witted. All his life, he has dreamed of becoming a great knight like the father he never knew, so he is devastated when he is rejected by Castle Redmount's Battleschool. Instead he is apprenticed to Halt, the mysterious Ranger whose ability to move unseen is thought to be the result of black magic.