Well done, OP.
Your school has had the perfect opportunity to teach boys that they need to be kind but fucked it up. Instead of being a shining example of how to handle this type of situation well they've taken the lazy route and piled all the responsibility on to little girls who are too young to understand that they can say no if it makes them feel uncomfortable even if they don't yet understand why they're uncomfortable. And once this boy has been accepted there's no going back when they're older and the self-consciousness of puberty really kicks in.
Young minds are so malleable. Most of the girls probably think the sun shines out of teacher's arse so if teacher 'suggests' in some way that it's the next best thing to flying unicorns to pander to a boy under the guise of 'kindness' they'll happily follow. I'm not a teacher but my best friend was and this was more or less how she explained what she called 'taming the herd' to me.
I'm shocked at the parents of one girl for not agreeing to you. The mum I could maybe understand but the dad? Surely he has known enough boys/men in his life to know that boys generally aren't kind and that a lot of males will have sexual feelings even in young puberty that they don't understand? The decent males in my life have always wanted to protect me from the males who are not so decent.
Unfortunately for me, they didn't always succeed and that's why I'm so hopeful that you succeed in your quest to not have a boy changing in with the girls. This poncho bullshit is just that. Give the boy the poncho and send him back in with the boys - where he belongs. If he doesn't like it provide a screen for him. But he is NOT a girl so should NOT change with them.
Don't get me started on this boy's parents! We all want the best for our children but this pair obviously think their little precious is much more important than a bunch of girls. They should have worked with the school about how the boys should be taught to be kind and accepting not taking the easy option of dumping on the girls like this.
Putting a boy in a changing room full of girls even if he 'feels like a girl' and wears dresses is NOT a good idea for so many reasons. It's lazy and woke of the school and being implemented for those completely wrong reasons.
I feel for all those on here who have posted their experiences of early puberty (I was also 8 with breasts, body hair, periods and size 6 feet - which caused me problems back then and bad feelings that have stayed with me all my life) and abuse in an effort to help you stay strong and fight this, OP. We're all behind you.
Keep us updated.