Hard pieces take time, and every minute student singers spend not knowing the piece is a minute they spend forming judgement on the piece. This can be perilous.
We all form judgements before we really understand pieces, and if a strong negative opinion is formed simply because the piece is hard to learn, that can be really tough to overcome.
The best strategy I’ve got for combatting that is to find the part that will grab the singers quickly. Learn that as fast as you can, and then use it as the “way in” to the rest of the piece. I’m currently working on a challenging piece in a fast 6/8, in minor – lots of pitfalls! But there’s a piece that modulates to the major and metrically modulates into 3/4…that section is much easier to learn (singers have an easier time feeling 3/4) and gave them a taste of what it will feel like to sing the piece.
Don’t start with the hardest part, though you might be tempted (because it will take the most time); start with the part that will grab the students.