Dr. med. Lehnert was elected as AStA chairperson in Bonn in 1956. He was part of the silent march with the DGB against the intervention of the Soviet Union in Hungary, founding Hungarian Aid, which was realised together with WUS. On April 21, 1958, he presented a study on urgent reform of student health care and proposals for its reorganisation. In November 1960 he participated in the meeting of the Executive Committee of International WUS in Geneva on behalf of Prof. Elbel. The main topic was "Student Health Facilities in Southeast Asia. In 1961 he joined the International Conference on Student Mental Health in Murten, Switzerland, organised by WHO, WUS, UNESCO; in 1962 he was at the Colombo, Ceylon Conference on Student Health in Southeast Asia and in 1962 at the "Help for Self-Help" international conference in Sooksu near Istanbul/Turkey.