CAN A PRIVATE-SCHOOL TEACHER OR DIRECTOR PROHIBIT A MUSLIM STUDENT FROM WEARING A HEADSCARF?
Summarized: A private-school teacher or director is generally not allowed to prohibit a Muslim student from wearing a headscarf. There could, however, be a neutrality regulation in the contract signed between the school and the student (or more specifically, her parents) which could prohibit the wearing of a headscarf.
Wearing a headscarf at a private school is dealt with differently in comparison to a public school (see here for the legal situation at public schools). A private school is not a state institution, so that in the case of a headscarf ban by the school, it cannot be seen as an interference on the part of the state in the religious freedom of the student. This scenario has more to do with two private individuals, both of whom can assert their fundamental rights against each other.
Before the enrollment process, a civil contract would be signed between the two parties, which would lay down the rights and duties of the school and the student. As long as the contract in question does not rule out the wearing of the headscarf (for example because of a neutrality requirement), it is not permissible for a teacher or director to do the same (for the question regarding whether or not contract clauses may pronounce a ban on the headscarf, see here).1
1 Regional Court Bonn, decision from 20.03.2015,1 O 365/14, margin 42.