They're talking about the division between Gymnasium, Realschule, and Hauptschule. It's actually state to state nowadays whether they have separate schools or Gesamtschulen, but I understand even in Gesamtschulen, in many Bundesländer there's some internal separation.
Where are you in DE, that this is unknown to you? In Köln just 15 years ago I knew parents who had the horror scenario: a 4th grade teacher who quietly believed that girls shouldn't go to university. They switched their daughter schools that year.
