Except many parents don't get a choice about where to send their children.
Fighting injustice is not just about the big things world changing things, the little things are often just as, if not more important.
When I speak to my kids or nibblings about something they think is unfair, I get them to think about why the rule is in place. Sometimes they come to the conclusion that they don't like it but understand the reason for it.
If they still think its unfair, I talk them through how they can challenge that rule in a way that is likely to get results. If its a school rule, I encourage them to ask to speak to the relevant teacher, or join the school Council or whatever where they can make the argument for change.
I also make it clear to them that they should do it in the right way e.g. not be disruptive in class, and to either try a different way or suck it up if they are not successful in their campaign.
To say school policies are set in stone is patently ridiculous and just complaining for the sake of it is not a valid response. But effecting change by pointing out unfairness or the benefits of change is more than valid.
Sure, getting changes made to a uniform policy so that kids can wear more comfortable shoes, dye their hair purple or get a second set of piercings isn't going to change the world, but it is going to teach children how to successfully effect change through conversation, debate, collaboration and using logic and evidence.
And those children will take those skills into an adulthood where they may very well end up using them to effect bigger, history making changes.
Or we could just teach them to wear what they are told, ask permission to regulate their own bodies, to not dare show any individualism, to accept without question what someone in a position of power over them says...I'm sure there is no harm in that at all 🙄