Why should teachers be permitted to display overtly political messages in what should be an apolitical learning environment?
Well, the simple answer is taht a teachers politics should have no bearing in the teaching of pupils, for there is a great power inbalance in the teacher to pupil relationship.
In general, pupils are not that experienced in many ways and so are very open to influence and teachers are there to teach and not express their views and opinions on vulnerable pupils. A balanced view needs to be given in all situations, so that pupils are allowed to gain their own views and opinions without any due influence, for pupils are gaining information from many sources and some that are very difficult to control, such as social media and some other news medias. But, teachers are just one part, even though in pupils pre 18 years are there for a large part of their pupils lives only exceeded by the time spent with parents and maybe siblings and other close pupil to pupil relationships.
Teachers are there to provide facts and theories, but not personal opinions, as because of the power inbalance of teachers to pupils undue influence can easily be given, innocently, lets alone deliberately.
Source: Teachers Should Put Their Political Flags Away At School