It is "The young magician" (original Polish title "Cudowne dziecko").
A 12-year boy feels left out by his classmates because he has no interest in athletics. He finally discovers his calling when his parents take him to a magic show. Picked by the magician to be his helper during a trick he is drawn into the world of magic and wants to learn more. He becomes proficient in tricks and also discovers his telekinetic powers. Unfortunately that further alienates him from his peers and his parents are afraid of his powers. Things change when a national emergency arises and the government asks him to help.
I've seen it as a kid myself :) I remember the scene when he longingly looks through the window on his classmate's birthday party (that he wasn't invited to) and accidentally makes all soda cans to explode.
Here is the fragment with throwing flowers at his friend: