I'm looking for a book I read in late elementary school, early secondary that is somewhere in late 90s. This was polish translation, so book is probably a bit older. I can't remember much but there are few details that stuck in my mind. There was a boy, a wizard apprentice, which was learning basics of magic, and was shown how to possess body of an animal. Possess is maybe not the best word, it was rather exchange. On one occasion he possessed a bird (probably a hawk) and was distracted by pleasure derived by flight, and did not return to his body in time. He was locked in birds body for some time and was later rescued by his master at the last possible instant. The change in bodies would become permanent, but caught in time It was said that this had no ill effects on long time-scale other than that the boy acquired taste for raw meat, and bird was thought to experience strange dreams about walking on foot.
If I recall correctly magic in this book was very well defined, it had extremely strict rules. You could die lifting heavy stone because it exerted heavy force on your mind, so you had to come up with some tricks like levers (magical ones, not real). But this magic part may be out of some other book.
This was probably first book of some cycle.
The boy probably had a friend his age, but this is all so lost in time...