Bridle the Wind - Joan Aiken - Bog

After visiting his family in England, Felix is on his way back to Spain when he's shipwrecked off the coast of France. He is taken in by monks to recover from his ordeal--but it soon becomes clear to him that he is actually being held prisoner. Felix encounters an injured boy, Juan, on the grounds of the monastery and saves him from death. The two boys escape and continue on to Spain together--but a gang is pursuing Juan, and the journey is more dangerous than they imagined.