Take a look at the size of the rock that the boys push onto Piggy. A rock of that size, falling from that height would've exploded Piggy's head like a ripe melon. It also would've crushed his body into a mangled heap.
The attempt to make the locations look wild and uninhabited were obviously not altogether successful. Near the beginning of the movie, some boys are sitting in a sandy clearing in public-school blazers. On the right of the screen you can clearly see a tent peg with ANTONY M written on it. Also, as one of Jack's choirboy-hunters thrashes through the undergrowth, look closely and you can see that he accidentally impales an old beer can.