Some of the best known schools for folk music and ethnomusicology over the years have been U Penn and Indiana University.

Here is U Penn's page of musicology links, which includes links to some colleges.

I don't know what those programs are like nowadays (though I know Indiana has Latin American and early music departments).

If it were me doing the going back to school thing, I would find out who was teaching what I wanted to learn, and then go wherever it is they are teaching, if possible.