Here's why: British school boys are first, British. The stereotype of the British is that they are highly civilized and play by the rules. Golding might argue that they are therefore the least likely to fall victim to primal urges and become savage. He chose school boys of various ages, but all under the age of recognizable moral maturity because he wanted to present the argument that without societal constraints, we are all savages; if he had used adults, adults would behave more rationally--at least for a while. He also excluded girls because, in his own words, he did not want "sex to rear its ugly head." He wanted nothing to interfere with his vision of the basic instincts of humanity.
One can assume that the main reason Golding used British school boys is that he was himself British. He had attended the sort of schools that these boys were attending, and he was a schoolmaster in the British system. He had been through this system that was supposed to produce civilized young men, and he was now part of it, and both perspectives gave him lots of chances to observe just how wild and uncivilized these boys were, and just how close to the surface their basic savage nature could be found.
In a general sense, the British were supposed to be civilized, and had for many decades thought of themselves as carrying civilization with them as they went into foreign lands and ran their empire. The book is an extended commentary on this ideal.
For more on Golding's background, see the enotes biography on him (available via the link below).