… Curtis Institute of Music.
Per this chart on eCollegeFinder, Curtis accepts just 5 percent of its applicants. This 5-percent rate would make it the most selective school in the country, edging out Harvard and Stanford (both at 6 percent).
Data from the class of 2012 in U.S. News and World Report ranked Curtis the third-most selective college in the country, just behind those same two schools. (The other selective schools are about what you'd expect: The Naval Academy, Cooper Union — which was, at the time, free — Yale, Julliard.)
Per an article in the Daily Pennsylvanian last year, Penn's admit rate for the class of 2017 was a record-low 12.1 percent.