A thought is a thought no matter who thought it up and quoted it. A truth is a truth no matter who says it. An interesting or possibly useful concept is just that no matter who was the original author or source.
So MLK was a deeply political person. So Ronald Reagan was a deeply political person. Charlie Kirk offered commentary on a great many different topics and themes, some political, some not. MLK and Reagan were pretty much all sociopolitical in their public lives..
But MLK and Reagan non political quotes on posters in that classroom were just fine and the teacher was not required to take them down. Only the Charlie Kirk quote was objectionable. And why? Because he is the assigned boogie man to trash and misrepresent by the left right now. Nobody much cares what MLK or Reagan had to say these days.
As I posted earlier I have no problem with a Karl Marx quote that is promoting a concept, a truth and he had many along with his more squirrely and dangerous sociopolitical views.
Education is much much more than just feeding the students facts they are supposed to remember for the test. The teacher who encourages the students to consider all sides of the issue, to think critically about the probable and possible consequences and effect of cultural trends, political policy, and everything else that affects our lives toward a goal of arriving at the best possible truth or conclusion, is giving his/her students a great blessing.
Posters that encourage students to think is just one of many ways to do that.