It's mandatory for students to wear white at the graduation ceremony. If you don't wear white, you won't be allowed to walk across the stage and the school will simply mail you your diploma. What if a student can't find a white suit or dress? You can't take away a huge moment in their life (graduation) just because of it.
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What kind of school does that? Maybe ask your parent to write a letter to the principal or school office. Schools usually listen to parents.
MS
Every school that I know of with a dress code for events will absolutely work with students who have difficulty finding something appropriate to wear. It seems to me there is plenty of notice about this, so it shouldn't be too much of a hardship for anyone. I have seen that it's common for schools to require white to be worn under white graduation gowns because anything else shows through; I've never heard of the requirement otherwise.
Anonymous
You're wearing a graduation gown from your neck down to below your knees, so you could go completely naked underneath and they wouldn't know, meaning all you really need is a pair of white tube socks pulled up to your knees. I think literally anybody who doesn't already own a pair of white tube socks can afford $1 at Dollar General for a pair of white tube socks.