All physical education should be separated by sex.
I agree. It would smooth a lot of issues over.
The UN Universal Declaration of Human Rights does not.
The UN Declaration of Human Rights also says healthcare, housing, and employment are rights, which they are not. While I agree with Article 18, I would rather argue the American (or in this case German) constitution's guarantee of religious freedom then the UN Declaration, which I don't view as legitimate.
Anywho, I think this argument depends on the school. If it is a private school in question, then the students have the right to withdraw from the school or comply. If it is a public school, which it is, then I don't think the state should roll over religious freedom for the sake of P.E.