I was having problems with school choir, because it gives no indication of the high quality that is to be expected of an escolanía, the choir attached to a Choir School, ie a school specializing in music and singing. So I decided on a different tack. the members of a choir school choir (the repetition is intended) are called choristers, whereas you wouldn't use the word for members of a run-of-the-mill school choir.

http://www.choirschools.org.uk/2csahtml/chorister.htm