The school (teachers and students together) recognizes an ideology (patriarchy) that they see as a problem and responds by creating a culture that opposes that ideology, in order to reduce the power of that ideology. I don't see any way to take a neutral stance on this issue, as ``doing nothing'' is supporting the status quo (patriarchy), so if you agree that patriarchy is a problem, then the school should be celebrated for picking the right side of the issue. If you like patriarchy, that's a different issue.

So basically, anyone who disagrees with you is a racist/sexist?

In isolate, this is an absurd sentence. If I said "I don't think we should [do horrible thing to group x]", then you could easily retort with "yeah, but if I disagree with you I'm against group x?" in a "screw you" sort of way, and, well, it'd be true, wouldn't it?

I'm not saying anything either way about the GP, but I'd suggest it'd be better to tackle the point, rather than have such a knee-jerk reaction.

Well, if racism and sexism are real (they are) and person A believes that these are bad, then if person B disagrees with person A, that means person B believes racism and/or sexism are not bad.

Language like "racist/sexist" suggests that it's a binary, that each person is either racist or not racist. It's more useful to recognize that the dominant culture in the US is racist and that people benefit from/support that culture to varying degrees. Person B supports it.

You can reject (or at least question) the premise.

Do you mean the premise that the patriarchy exists?

Perhaps the premise that the school's actions successfully (does the policy work?), appropriately (is the policy ethical?), and accurately (does the policy have negative side effects?) counters that ideology.