The professors organized a net of secret high schools, trade schools and special courses on forbidden subjects, such as the Polish language, history and geography. A special case were the secret talmudic schools organized in ghettos. Until 1944 there were more than a million secret high school students in Poland. At least 18,000 students passed their final school exams and received their certificates. This led to a bizarre situation in which students of formally non-existent high schools entered formally non-existent universities. Most of these certificates were issued on pre-war forms with the dates forged to indicate either 1938 or 1939. These were later accepted by post-war Polish universities.