Bismarck had the support of the traditional landowning class in Prussia (the “Junkers”). He favored the continued dominance of this class, and his implementation of portions of the welfare state were a strategy to co-opt the growing socialist movement. By promising social “insurance” he reduced the risk of social unrest from the working class. So he was neither a socialist nor a classical liberal, although he obviously displayed more traits of the former because of his statism.
It sounds like your teacher gave what was in most respects a correct answer.