I don't know why people have this notion that teens will magically want to start performing and singing in parts when the boys' voices begin to drop around ages 13-14. The poor boys that don't already sing hardly have the ability to make it gracefully through that awkward stage of life. And girls that aren't singing through their childhoods don't fair much better. Too many youth never find their "head" voice placement and fail to learn how to match pitches beyond their speaking vocal range, let alone develop beautiful tone or breath control or sight reading skills.
The best way to get teenagers prepared to be life-long singers is to give them lots of experience singing while they are children.
1. SING at HOME! Sing for fun. Sing while you work. Get them off their devices and sing in the car! A cappella! Take them with you to Ward Choir. Convince the director to sometimes do a "Family Choir" that features parts for Children's voices.
2. Encourage them to join CHORUS or BAND at school.
3. SING in PRIMARY, and please don't sing songs out of their natural singing range which is Middle C up to E above Treble C. Allow them to practice vocalizing in their "head" voice placement.
After all this preparation, the teenagers may actually be willing to sing in choir -- because they can!
Working with youth who have little previous exposure to singing and really don't know if they can sing takes MUCH MORE EFFORT!
I have had that opportunity more than once, believe me! The first time I was assigned to prepare a Youth Chorus to sing for Stake Conference in Baltimore, I just assumed the vast majority of youth had never learned to sing in parts before. We were asked to sing "True to the Faith" and "Come We That Love the Lord." I personally went to each unit to practice with the youth. Fortunately, we had good support from our leadership in each unit. Their Branch and Ward music people also practiced with them. I allowed several months to get the preliminary work done. Then we had a few practices together as a Stake. Guess what, the hard work paid off! The kids did great. (I did think it best to remove them from their phones during Conference, though. Our High Council Advisor guarded the phones in a box.)
After this big effort, I sure hope the Ward leadership continued on their success. Success breeds success. But you've gotta keep the ball rolling!!!!!