Well, since these are kids into Labor Party politics, they’re probably not super aggressive. Nor are they used to guns. So running away and/or hiding is a more natural reaction. And running away is a good survival instinct much of the time — it’s just that on a small island it’s not very effective. Also, that police uniform clearly confused everyone when it came to reacting.
Finally, while it’s a small island, it’s got a lot of trees and a rocky shoreline. That would make it difficult to see what was going on from a distance, thus making it difficult for people to figure out exactly what was going on besides hearing shots and screams.