I used to scare people because I was the only one in the junoir (priamry) school who could reach certain notes (even if it was just a C or a D - I go nearly an octave above that now.....) and they would look at me. So I stopped. Maybe everyone is afraid of making that extra step into volume and pitch because they don't want to be heard getting it wrong. Try a few scales to warm them through, and you can get an idea of who is most likely to hit it, the near misses and those who need to be alto/bass.

Or try slippng in a ringer - someone who knows the piece, hits the notes and isn't afraid of funny looks. Once there is someone else up there, they may decide it isn't so bad after all.

LTS - AKA the ringer......