Berlin and the western state of North-Rhine Westphalia have just announced that boys and girls could be separated for science and information technology lessons from April.
At least three other German states are expected to follow suit. The decision came in the week that the Third International Mathematics and Science Study found boys outperformed girls in every country in these subjects. Berlin and North-Rhine Westphalia have conducted a pilot project since 1990 allowing single-sex lessons in specific subjects but for no more than six months at a time.
"At the time we were led by gender research that showed quite clearly that girls have more faith in themselves in lessons if the boys are not there," said Sybille Volkholz of the Green party who was then Berlin's senator for schools.
Even before 1990 there were separate classes for biology and sex education. But the main impetus for single-sex classes came with the introduction of information technology when girls "constantly lost out in the competition for access to a computer", says Ulrich Thuenken, of Berlin's Ministry for Culture. Some schools introduced special six-month courses for girls in computers.
Under the pilot project, 15 per cent of Berlin schools - or more than 150 schools, 75 of them primaries - taught the sciences and sometimes mathematics in single-sex classes for a term at a time. In North-Rhine Westphalia geography was included.
Some teachers now believe modern languages should be taught separately to benefit boys who tend to be weaker in language-learning.
The results of the pilots are undisputed. According to a report of the North Rhine Committee on Co-education: "It is more fun for boys and girls to be taught separately in certain subjects."
A similar report by the Berlin authority noted that "separate classes has strengthened the confidence of girls".
Even opponents have focused on whether single-sex classes should be allowed in primary schools and in what subjects, rather than on the principle of separate teaching itself.
Berlin's senator for schools, Ingrid Stahmer, announced that separate lessons could go ahead and could be of unlimited duration. The state will also give schools more freedom to decide which subjects should be taught this way.
But Mrs Stahmer denied that the move meant an end to co-education, which would be a political hot-potato in Germany.
Many parents remember when boys' schools prepared pupils for grammar schools and university, and girls' schools taught domestic and secretarial skills.
However, even the smaller step of allowing separate classes is a recognition that co-education has failed girls, dampening their enthusiasm for scientific and technical subjects.