My daughter came to us on the first day of school telling us that one of her teachers had a cross and a Bible quote on the wall. I didn’t want to get my daughter into trouble by letting the principal know about it myself since they would attach my name to it and she might face retaliation. So I wrote to the FFRF, sent them a picture my daughter drew of the cross and the Bible quote (no phones in class, so she couldn’t get a photo).
Not only were they responsive quickly when I contacted them, within a few weeks, they sent an email to the school superintendent with all the relevant legal information as to why this teacher needs to take the cross and Bible quote down and without identifying who reported her.
I’m not a member due to some financial issues right now, but as soon as those clear up, I am going to join. They do good work.
Consider joining yourself: https://ffrf.org/
I’d also be fine with it in the context of a historical lesson or a survey of world religions. It’s rather difficult to discuss history frankly without discussing religion’s outsized role in a lot of it. So perhaps an image of a knight riding under a cross wouldn’t be out of place, so long as it was properly contextualized.