Continuity mistake: The ninja with the tattoo that attacks Jack is a large built man with olive skin. When Jack sees the tattoo on Kai's teammate, he is now a slight built, Caucasian teenager.
Continuity mistake: The kids fall to the ground after they are eliminated from the game but when Jerry takes out a knight and the wizard along the path, only the knight is seen lying down in the subsequent shots; the wizard is nowhere to be seen.
Answer: It depends. If you watch the action scenes in slow motion, you can often tell if it's the actor or a stunt double. Leo Howard seems to do a lot of his own fight scenes and acrobatics while many of the fights and flips by other characters are done by doubles; although occasionally Olivia Holt will perform a simple flip or fight.
Both Leo Howard and Olivia Holt have black belts and completed their own stunts.