It isn't likely that any single flash would evenly illuminate an entire gymnasium. I would suspect she probably used available light.
From everything you have indicated, it sounds like the flash you purchased should work just fine. You're going to have to try it sometime. What are you waiting for?
As far as the picture is concerned, I don't like having the tree trunk behind only part of his head. It creates an unnatural split in the picture. I don't like how there are branches "growing" out of his head. You might have improved the picture by using the macro mode so that you could get better control of the depth of field.