Yes, it could be that the choir director is a white supremacist. Or it could be that there are cultural and/or class reasons why the ethnicities which make up the majority of the school are so unrepresented. I have known a minority of Muslims of certain nationalities to take their children out of music lessons, and there may also be a sense that school choirs sing church music. Some children will have religious studies after the school day, and won't want a regular after-school commitment which might clash.
Especially if there is a high percentage of immigrant families in the school, there may well be an emphasis on 'useful' after-school activities/chores/tutoring, rather than what might be perceived as white, middle-class frills. I have close Indian immigrant friends who are regarded as eccentric by their relatives because their children have piano lessons and play football in kids' leagues rather than have extra maths and English tutoring.
Some more conservative families of various backgrounds don't like girls to be out and about after school hours, and some very much socialise within the family, so the kids are either at school or at home, or with cousins at a family member's house.
It may not even be that. I went to an all-white school in the 1970s/80s with a small minority of girls from middle-class backgrounds, who were hugely over-represented in the choir/drama/inter-school public-speaking and talent shows, purely because their parents encouraged them to audition and to be confident, and they often had out of school dance/music/elocution lessons so were far better than the rest.
I only discovered after leaving school that I had a good voice, because I would sooner have thought of trying to teach myself to fly than audition for the choir.