Allowing overt displays of religion by teachers in our secular schools, would open the door (by virtue of the 1st and 14th amendments) to any and all belief systems to be put on display of the person of any teacher. The government could not discriminate against any faith or non-faith beliefs and so any symbols advertising the teacher's belief system would become allowed by default. Some of which might be HIGHLY objectionable to those who imagine teachers displaying mainstream belief symbols.
Unless it falls under certain unprotected categories such as Obscenity, Indecency, or Libel, teachers already have this right. Currently, they are allowed by default and this is not a major issue.
Of course it should be allowed, that is the definition of freedom of religion. We allow many other religions to display it outwardly such as turbans and burkas. There are no negative effects on the students. Other than the students will feel free to believe as they want. Infringing on a right to express religion would only teach students that they don't have the right to practice their religion.