The cis kid who was cast to replace the trans kid should refuse on principle.
The whole cast and backstage staff should pretend to go along with it until opening night, and then on opening night as the curtains open, they make a short statement about the injustice then walk off stage and go home.
Alternatively, the cast should covertly learn the role of an opposite gender cast member. Then on opening night, the cast members assume their opposite role and perform the whole play ignoring the costume inconsistencies.
As a former theater kid (and queer person) this is close to the right answer. The cis replacement should show up on day 1 and do the most over the top, flaming, fabulous Curly the world has ever seen.
You're right, it is a speck, so why the fuck is the school board singling out a child to punish over something completely insignificant? Do you think that child is going to grow up with a strong respect for authority, knowing that they will ruin their prospects over trivial bullshit?
Just because it doesn’t have an effect on you personally doesn’t mean it’s a speck. Other human beings exist and have feelings. Maybe stop being a narcissist.