The audience doesn't actually witness the suicide, yet the scene is still powerful. What factors in the film combine to make this scene so effective?

Director Peter Weir makes good use of montage, soundtrack and silence for the suicide scene. Neal puts his crown on (used by Puck) which representing Neal as a Christ-figure. The montage flows quickly leaving the audience with little time to process what might have happened. The scene finishes with the boys telling Todd Anderson (Ethan Hawke) of the tragedy. Leaving it to viewers imagination is much more potent than showing it.