I assume you mean general content (including pride flags and other imagery) that reference queer and nonbinary people.

You could think of it this way: we already have implicit acceptance everywhere of straight cis students and staff, and that's not considered inappropriate. Straight and cisgender is the norm and the expectation of our society, and no additional signalling or visibility really needs to be present.

But queer and/or nonbinary people are not aligned with that societal expectation, we are suppressed by it. The late great Elite Wiesel's famous words come to mind: "We must take sides. Neutrality helps the oppressor, never the oppressed. Silence encourages the tormentor, never the tormented". In this way, the so-called neutrality of the policy banning any expression of social politics oppresses queer people, because representation must be had for equality to thrive.

Additionally, consider this: do we really want children to be raised in an environment where they're never exposed to what people unlike them think? How can the answer to a fear of indoctrination be to hide them from anyone else's opinions, rather than showing them the breadth of the world and teaching them how to evaluate and make good decisions? I can guarantee you this, it's the fastest path to indoctrination to never be exposed to anything different. If parents are truly that concerned that their kid is going to get diverted into some sort of cult of thought or personality, they're welcome to home school their kids. Exposure to queer people is not harmful.

I can appreciate a desire to stop teachers and staff from inappropriately expressing their personal opinions, especially in front of the kids, and especially as employees of the government. But we already express the messaging that "straight = accepted" all over the place, all day every day. What messaging is present when we pair that with suppressing expressions that "queer = accepted" as well?