Nobody said that these had to be in the States, did they? If you wanted to try the UK, how about Folk South West's "In the Tradition" Bath Easter school, held on the premises of Bath Spa University, with professional tutors on Song, Fiddle, Guitar, band, harmony etc. I've done the Traditional Song course three times, first two with Shirley Collins and Eddie Upton as tutors, third with John Kirkpatrick, Eddie Upton and Nancy Kerr. Tutors on other courses in the last few years have included Martin Carthy, Tim van Eycken, Paul Burgess, Chris Wood, etc. Courses cover four days, and are on the basis of masterclasses, with one-to-one tuition in front of your group.
There are also the Folkworks Summer schools - one for adults and the other for young students - held in Durham in August. I've never been on one of these, but understand that they follow similar principals. These courses last a week, and the tutors include similar luminaries of the folk world.