Like many promising students from modest backgrounds, Nolan Arkansas started his college search with a major barrier: He was unaware of all of his options and undervalued his own potential. He figured he was bound for a public university in his home state or just across the border at the University of Tennessee. Elite private schools seemed inconceivable. But a growing movement of nonprofit talent hunters and advisers is seeking to raise the ambitions of disadvantaged students and connect them with premier colleges, attacking a widespread problem researchers call “undermatching.”