Question: Do the actors perform their own stunts on this show? If not, they did a great job of casting look-alikes.
Question: Did Leo Howard actually do all the skateboarding tricks that are in the show or were they stunt doubles?
Answer: Stunt doubles.
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.