So, they want to keep the picture up because it's been there for a while and because it isn't hurting anything. The thing is, both of these are rationalizations to justify keeping the status quo intact, without addressing the actual issues of having this picture hanging in a public school.
First off, it simply doesn't matter how long the picture has been up there. Whether the picture was put up yesterday, last year, or a century ago is immaterial. What matters is whether it's appropriate for the picture to be up on the wall in the first place, not how long it took before someone complained about it.
Second, you can't judge how harmful something like this is by asking people who aren't affected it. Imagine if we decided whether a law was discriminatory based on how it affected people not targeted by that law? Like, say, deciding on a law about poll taxes or literacy tests based not on how it affected black people (who were the targets of such laws), but how it affected white people (who were usually exempt from these kinds of laws). It would probably be considered a travesty of justice by most people, and rightfully so.
Yet this is exactly the principle that the superintendent is proposing we use - that we judge how harmful it is based on the reactions of Christians, rather than non-Christians.