Took a while but I finally got some feedback. Turns out the federal state of Brandenburg has rejected the application Opus Dei has made for the foundation of a privately run school.
The official reason provided is not quite as clear a statement as I might have hoped for, but I won’t start nitpicking here. This is how it goes:
Article 7 of the [German] Constitution establishes that private schools, as an alternative to public schools, are subject to governmental supervision and thus to the laws of the federal state in question. The federal states independently govern educational objectives in their own state constitutions and school laws.
According to § 120 of the Brandenburg school law private schools serve as an alternative to public schools present or intended in the state. The application of the society for the support of schools of free agencies, however, is no full-featured alternative because it exclusively relates to the education of boys. Public schools in the state of Brandenburg, however, are coeducational in accordance with § 4 of the Brandenburg school law.
This regulation for coeducation implements the requirement of § 3 of the Brandenburg school law, according to which schools in the state are to be arranged in a way to “ensure equal access independent of economical or social condition, national origin, political or religious conviction, or gender.”
This is based on the sexual equality laid down in the state’s constitution. There § 12 establishes that nobody may be favoured or disadvantaged because of his gender, and that the state is obliged to ensure the equality of man and woman in education “through effective means.” This includes the equal right of access to educational facilities.