Is the head of the scientific club considered responsible and sinful for the mixing that occurs between sexes at the university? And what is the temporary solution until he is able to withdraw from the club?

If you can prevent unlawful mixing (ikhtilat) in the club, then advise the students and try to establish a system that conforms to Sharia (definition) by preventing khalwa (definition) (private seclusion of a man and a non-mahram (definition) woman) and by designating specific times and places for men and women. Emulate the Prophet, peace and blessings be upon him, who commanded women to keep away from the middle of the road and designated a special door for them in the mosque. However, if you are unable to prevent mixing, then resign, unless your presence mitigates the evil, in which case, be patient and continue your efforts.