The two schools are practically neighbors — just three miles apart — but Newton South and Brighton high schools might as well be on different planets. Newton South is a sterling suburban high school that routinely sends graduates to the country’s elite universities. Brighton High is a floundering Boston school where fewer than 30 percent of graduates earn a college degree or other credential within six years of graduation. Globe reporters spent weeks last spring at the two schools — both public — to try to understand how they could produce such staggeringly different results. What were the obstacles to success? What were the suburban advantages? What could be done — and how much would it cost — to bridge the yawning gulf that divides them? The answers go far beyond money, touching on some of the deepest inequalities in our society.