I know that most boarding schools have choirs or choruses, but which ones are known for their strong choir/chorus programs?
Maybe NMH?
SPS has a great choir program, but I don’t really know how it compares to other schools’ programs.
I’ll second SPS.
SAS has the Choral Scholar program/group. It’s a smaller school, so the group is smaller, but to my ear, very strong for a high school ensemble.
https://www.standrews-de.org/arts/choral-music-and-voice
If student was truly interested in voice (pre-professional)…maybe something like Idyllwild Arts?
I know nothing about music and high school (well, at that level). Are there any competitions/audition festivals to see which schools routinely send kids/groups?
I usually laud Mercersburg, but this is an area where it falls short.
I think I was literally typing you a personal message while you wrote the above, @twinsmama! Has your daughter seen or heard other schools’ choruses?
I sent you a PM…for others, it is really that Mercersburg’s choral program is nothing special, and my daughter came from a much higher level program in 9th grade. However, there is a new choral director this year, so things may change.
From what I just heard in the Christmas service, it seems the new choir director is making a huge difference. And isn’t it kind of sad that my first thought was to go to CC and post about it? 
Most boarding schools allow/encourage students to take private voice lessons. As the student body turns over a quarter of the students each year, I would not judge a school by any particular year. As far as I know, the boarding schools that aren’t dedicated to the arts don’t recruit for vocal talent. And anyways, for many boys, their voice in 8th grade will not be their voice in 12th grade.
I recommend you look for a choral director whose style you like, as well as good voice teachers. Things to consider would be: what sort of music does the chorus perform? (Secular, Sacred, Romantic, Baroque, Modern, Jazz, etc.) Can I live with that sort of music? How many choruses are there? Are there other vocal performance opportunities, such as A Cappella groups? Are those groups student-led, or under the direction of the music department? Does the school put on a musical each year? Does the chorus travel?
I would personally prefer a school which encourages growth in musical students, rather than students who are perfect the day they arrive. As far as I know, there aren’t any competitive choral festivals in the private school world; our state does hold state competitions for vocal and instrumental performers.
Another question to ask: do any students from the school choose to enter conservatory upon graduation? Are any graduates participating in A Cappella groups or choirs at their colleges?
We were happy with St. Mark’s School’s choir. You can find a description of their current program, as well as recent samples of student performances, here: https://www.stmarksschool.org/academics/academics-programs/arts/music. St. Mark’s holds a two-day choral festival in the winter, the Wick Choral Festival. Groups from other schools and colleges are invited; it’s a different sampling of outside schools each year.
Schools with music programs will often post samples of student performances on their websites. If you are near enough to schools to visit, I recommend attending concerts, if this is a make-or-break feature for your family. The concerts are usually free, and open to all.
Choate has an excellent choral and vocal music program. There’s a chamber chorus and a festival chorus and several a cappella groups. The festival chorus travels to perform and I believe they go abroad every other year. Plus, if your student is a serious musician, he or she can try out for the Arts Concentration program, which enables the student to skip sports two out of three trimesters in order to have more practice time, and also requires some music theory and music history class work. There is an arts con program specifically for voice. Just as importantly, the arts are taken seriously at Choate and there is a warm community of artsy students so you don’t have to be an athlete to fit in there.
I don’t have a good answer here, but suggest you consider whether there are community resources, like a state honors chorus program or adult program in the community as an addition or alternative to the school program (and whether the school will support transportation to that alternative). The size of many schools and the fact that they don’t recruit in this area may make it difficult to find a school that otherwise fits that can meet the needs of an advanced musician.
Andover has a traveling Choir at multiple levels and have travelled to China, Spain, California, DC in the past couple of years.
NMH has multiple singing groups and the choir travels including, I believe, abroad. There is a long history of choral singing at the school. The most recent Christmas Vespers program is available on-line. I would ask to meet with the appropriate faculty person at each school when you interview (if it isn’t too late), or email them for more info.