Let’s see. Grace Lutheran Church in River Forest (just outside Chicago) runs a yearly Bach Cantata Camp for high school singers and strings. Weeklong, learn one cantata and enough other polyphony/psalms/hymns to do a vespers with cantata at the end. The orchestra and solo arias are filled out by pros from the Chicago area, giving a great chance to work with pros who also deeply appreciate church music. Housing on the campus of Concordia Universitt Chicago, next door to the church. Not seeing this year’s program on the website yet but you could email and ask. http://graceriverforest.org/bach-cantata-vespers/current-season/cantatas/
Credo Music https://www.credomusic.org/ runs several summer programs, by audition, for various instruments. They are run by an evangelical professor at Oberlin, but the camps are geared toward helping Christians to learn to be very good at what they do in order to praise God, not towards proselytizing. Faculty are outstanding and again some pros are brought in as soloists or to cover harpsichord or other unusual instruments not generally taught in high school.
The American Guild of Organists has for years run Pipe Organ Encounters, for students who are pianists interested in the organ, or organists not yet in college. Beginner and advanced programs are offered. These impacted me considerably in high school, and really help students to understand the way to go from enjoying organ music at church to becoming serious church musicians capable of holding a full-time position. https://www.agohq.org/education/poe/
I have participated in or worked for all of these and can recommend them wholeheartedly. The skills taught may not be exclusively used on, but are completely applicable to, the Catholic repertory, but high schoolers will not find better instruction anywhere else in the country.
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