A teacher should only be fired if he/she does not comply with the regulations set forth by the local ministry of education. If you ask me what these regulations *should* be, the partial answer is that in my opinion, public schools should teach children nothing except

1. Reading,

2. Basic math,

3. Basic computer and internet skills,

4. An overview of the political and legal system in force in their country.

Neither religion, nor science, nor arts, nor any other subject should be part of the public school curriculum. Beside the public system, there can be many private and non-for-profit schools, with different curricula, regulations and criteria for certifying teachers.